Display device, method for manufacturing the same, and electronic device

ABSTRACT

A liquid crystal display device with a high aperture ratio is provided. A liquid crystal display device with low power consumption is provided. A display device includes a transistor and a capacitor. The transistor includes a first insulating layer, a first semiconductor layer in contact with the first insulating layer, a second insulating layer in contact with the first semiconductor layer, and a first conductive layer electrically connected to the first semiconductor layer via an opening portion provided in the second insulating layer. The capacitor includes a second conductive layer in contact with the first insulating layer, the second insulating layer in contact with the second conductive layer, and the first conductive layer in contact with the second insulating layer. The second conductive layer includes a composition similar to that of the first semiconductor layer. The first conductive layer and the second conductive layer are configured to transmit visible light.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a display device, amethod for manufacturing the display device, and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. Examples of the technical field of one embodimentof the present invention include a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a memorydevice, an electronic device, a lighting device, an input device (suchas a touch sensor), an input/output device (such as a touch panel), amethod for driving any of them, and a method for manufacturing any ofthem.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquidcrystal display device and a light-emitting display device are formedusing silicon semiconductors such as amorphous silicon, single crystalsilicon, and polycrystalline silicon provided over glass substrates.Furthermore, such a transistor employing such a silicon semiconductor isused in integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which,instead of a silicon semiconductor, a metal oxide exhibitingsemiconductor characteristics is used in transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor. For example, in Patent Documents1 and 2, a technique is disclosed in which a transistor is manufacturedusing zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductorand the transistor is used as a switching element or the like of a pixelof a display device.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide aliquid crystal display device with a high aperture ratio and a methodfor manufacturing the liquid crystal display device. Another object ofone embodiment of the present invention is to provide a liquid crystaldisplay device which is formed through a simplified manufacturingprocess and a method for manufacturing the liquid crystal displaydevice. Another object of one embodiment of the present invention is toprovide a liquid crystal display device with low power consumption and amethod for manufacturing the liquid crystal display device. Anotherobject of one embodiment of the present invention is to provide a liquidcrystal display device with high resolution and a method formanufacturing the liquid crystal display device. Another object of oneembodiment of the present invention is to provide a highly reliableliquid crystal display device and a method for manufacturing the liquidcrystal display device. Another object of one embodiment of the presentinvention is to provide a novel liquid crystal display device and amethod for manufacturing the liquid crystal display device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isnot necessarily a need to achieve all the objects. Other objects can bederived from the description of the specification, the drawings, and theclaims.

One embodiment of the present invention is a display device including atransistor and a capacitor. The transistor includes a first insulatinglayer, a first semiconductor layer in contact with the first insulatinglayer, a second insulating layer in contact with the first semiconductorlayer, and a first conductive layer electrically connected to the firstsemiconductor layer via an opening portion provided in the secondinsulating layer. The first semiconductor layer includes a channelregion. The capacitor includes a second conductive layer in contact withthe first insulating layer, the second insulating layer in contact withthe second conductive layer, and the first conductive layer in contactwith the second insulating layer. The second conductive layer includes acomposition similar to a composition of the first semiconductor layer.The first conductive layer and the second conductive layer are eachconfigured to transmit visible light.

In the above embodiment, the first semiconductor layer, the firstconductive layer, and the second conductive layer may each include ametal oxide.

In the above embodiment, the metal oxide of the first semiconductorlayer may include one or more of metal elements included in the metaloxide of the first conductive layer.

In the above embodiment, the first conductive layer may include an In—Znoxide.

In the above embodiment, the second insulating layer may include siliconoxynitride.

The above embodiment further includes a third insulating layer incontact with the second insulating layer and the first conductive layer.The third insulating layer may include oxygen at a higher proportionthan a stoichiometric composition.

The above embodiment further includes a liquid crystal element whichincludes a liquid crystal layer and a pixel electrode. The pixelelectrode may be electrically connected to the first conductive layer.

In the above embodiment, the resistivity of the liquid crystal elementmay be greater than or equal to 1.0×10¹⁴ Ω·cm.

In the above embodiment, the frame frequency of the display device isgreater than or equal to 0.1 Hz and less than 60 Hz. The display devicemay be configured to retain data when the transistor is turned off afterthe data is written to the capacitor.

In the above embodiment, the frame frequency of the display device maybe greater than or equal to 0.1 Hz and less than 20 Hz.

The above embodiment further includes a scan line which is formed usinga metal material. The scan line may include a region overlapping withthe channel region of the transistor.

One embodiment of the present invention is an electronic deviceincluding the display device of one embodiment of the present inventionand an operation key.

One embodiment of the present invention is a method for manufacturing adisplay device, including the steps of forming a first semiconductorlayer and a second semiconductor layer; forming a first insulating layerso that the first insulating layer is in contact with the firstsemiconductor layer and the second semiconductor layer; forming a firstopening portion reaching the first semiconductor layer, in the firstinsulating layer; forming a third semiconductor layer so that the thirdsemiconductor layer overlaps with the second semiconductor layer and iselectrically connected to the first semiconductor layer via the firstopening portion; forming a second insulating layer so that the secondinsulating layer is in contact with the first insulating layer and thethird semiconductor layer; forming a second opening portion reaching thethird semiconductor layer, in the second insulating layer; and forming apixel electrode so that the pixel electrode is electrically connected tothe third semiconductor layer via the second opening portion. Theresistance of each of the first semiconductor layer and the secondsemiconductor layer is reduced in the step of forming the firstinsulating layer. The resistance of the third semiconductor layer isreduced in the step of forming the second insulating layer. Theresistance of a channel region of the first semiconductor layer with thereduced resistance is increased after the second insulating layer isformed.

The first semiconductor layer, the second semiconductor layer, and thethird semiconductor layer may be formed so as to include a metal oxide.

The third semiconductor layer may be formed so as to include an In—Znoxide.

The first insulating layer and the second insulating layer may be formedby a CVD method with use of a deposition gas containing silane.

The deposition gas may include a nitrogen oxide.

Heat treatment may be performed after the second insulating layer isformed.

A liquid crystal layer may be formed after the pixel electrode isformed.

According to one embodiment of the present invention, a liquid crystaldisplay device with a high aperture ratio and a method for manufacturingthe liquid crystal display device can be provided. According to oneembodiment of the present invention, a liquid crystal display devicewhich is formed through a simplified manufacturing process and a methodfor manufacturing the liquid crystal display device can be provided.According to one embodiment of the present invention, a liquid crystaldisplay device with low power consumption and a method for manufacturingthe liquid crystal display device can be provided. According to oneembodiment of the present invention, a liquid crystal display devicewith high resolution and a method for manufacturing the liquid crystaldisplay device can be provided. According to one embodiment of thepresent invention, a highly reliable liquid crystal display device and amethod for manufacturing the liquid crystal display device can beprovided. According to one embodiment of the present invention, a novelliquid crystal display device and a method for manufacturing the liquidcrystal display device can be provided.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects canbe derived from the description of the specification, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional views each illustrating an exampleof a display device.

FIGS. 2A to 2C are cross-sectional views each illustrating an example ofa display device.

FIGS. 3A1, 3A2, and 3B are top views and a cross-sectional viewillustrating an example of a display device.

FIG. 4 is a cross-sectional view illustrating an example of a displaydevice.

FIG. 5 is a perspective view illustrating an example of a displaydevice.

FIG. 6 is a cross-sectional view illustrating an example of a displaydevice.

FIG. 7 is a cross-sectional view illustrating an example of a displaydevice.

FIGS. 8A to 8C are cross-sectional views illustrating an example of amethod for manufacturing a display device.

FIGS. 9A to 9C are cross-sectional views illustrating an example of amethod for manufacturing a display device.

FIGS. 10A and 10B illustrate arrangement and configuration examples ofpixels.

FIGS. 11A and 11B are perspective views illustrating an example of adisplay device.

FIG. 12 is a cross-sectional view illustrating an example of a displaydevice.

FIGS. 13A to 13C show a configuration example of a pixel and an exampleof a driving mode.

FIGS. 14A and 14B are a block diagram and a timing chart of a touchsensor.

FIGS. 15A and 15B are a block diagram and a timing chart of a displaydevice.

FIGS. 16A to 16D illustrate the operation of a display device and atouch sensor.

FIGS. 17A to 17D illustrate the operation of a display device and atouch sensor.

FIGS. 18A to 18E illustrates examples of electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that one embodiment of the present invention is not limited to thefollowing description. It will be readily appreciated by those skilledin the art that modes and details of the present invention can bemodified in various ways without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and a description thereofis not repeated. Furthermore, the same hatching pattern is applied toportions having similar functions, and the portions are not especiallydenoted by reference numerals in some cases.

The position, size, range, or the like of each structure illustrated indrawings is not accurately represented in some cases for easyunderstanding. Therefore, the disclosed invention is not necessarilylimited to the position, size, range, or the like disclosed in thedrawings.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film”. Also,the term “insulating film” can be changed into the term “insulatinglayer”.

In this specification and the like, a metal oxide means an oxide ofmetal in a broad sense. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor (also simply referred to as an OS), and the like.For example, a metal oxide used in a semiconductor layer of a transistoris called an oxide semiconductor in some cases. In other words, an OSFET is a transistor including a metal oxide or an oxide semiconductor.

In this specification and the like, a metal oxide including nitrogen isalso called a metal oxide in some cases. Moreover, a metal oxideincluding nitrogen may be called a metal oxynitride.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention is described with reference to FIGS. 1A and 1B, FIGS. 2A to2C, FIGS. 3A1, 3A2, and 3B, FIG. 4 , FIG. 5 , FIG. 6 , FIG. 7 , FIGS. 8Ato 8C, FIGS. 9A to 9C, FIGS. 10A and 10B, FIGS. 11A and 11B, and FIG. 12.

<1 Structure Example 1 of Display Device>

First, the display device of one embodiment of the present invention isdescribed with reference to FIGS. 1A and 1B, FIGS. 2A to 2C, FIGS. 3A1,3A2, and 3B, and FIG. 4 .

The display device of one embodiment of the present invention includes adisplay element, a transistor, and a capacitor. The display element canbe a liquid crystal element, for example. Although a liquid crystalelement is described as the display element in the followingdescription, the display element which is applicable to one embodimentof the present invention is not limited to a liquid crystal element. Forexample, a light-emitting element having a self-light-emitting functionmay be used as the display element. As the light-emitting element, it ispossible to use, for example, an organic electroluminescent (EL)element, an inorganic EL element, a light-emitting diode (LED), aquantum-dot light-emitting diode (QLED), or a semiconductor laser. Notethat an element with a combination of a light source such as a backlight or a side light and a transmissive liquid crystal element may beused as the light-emitting element. Alternatively, both a liquid crystalelement and a light-emitting element may be used as the displayelements.

The liquid crystal element includes a pixel electrode, a liquid crystallayer, and a common electrode. The transistor and the capacitor areelectrically connected to the pixel electrode. The pixel electrode, thecommon electrode, and the capacitor each have a function of transmittingvisible light. The visible light passes through the liquid crystalelement and is emitted to the outside of the display device.

The display device of one embodiment of the present invention includes aregion where the capacitor transmits visible light. Thus, the capacitorcan be provided in a display region. Accordingly, the aperture ratio ofa pixel can be increased, and the power consumption of the displaydevice can be reduced. In addition, high definition can be achieved inthe display device.

The transistor of the display device of one embodiment of the presentinvention preferably has a channel-protective structure. With thestructure, a channel protective layer of the transistor and a dielectriclayer of the capacitor can be formed in one process. Thus, thereliability of the transistor can be increased, and the manufacturingprocess of the display device of one embodiment of the present inventioncan be simplified.

The display device of this embodiment further includes a scan line and asignal line. Each of the scan line and the signal line is electricallyconnected to the transistor. Each of the scan line and the signal lineincludes a metal layer. When a metal layer is used for each of the scanline and the signal line, the resistances of the scan line and thesignal line can be reduced.

The scan line preferably includes a portion overlapping with a channelregion of the transistor. The characteristics of the transistor mightvary by light irradiation depending on a material of the channel regionof the transistor. When the scan line includes the portion overlappingwith the channel region of the transistor, the channel region can beprevented from being irradiated with external light, light of abacklight, or the like. Thus, the reliability of the transistor can beincreased.

A display device 10A illustrated in FIG. 1A includes a substrate 11, asubstrate 12, a transistor 14, a liquid crystal element 15, a capacitor16, and the like. A backlight unit 13 is provided on the substrate 12side of the display device 10A.

The liquid crystal element 15 includes a pixel electrode 21, a liquidcrystal layer 22, and a common electrode 23. The pixel electrode 21 iselectrically connected to the transistor 14 through an opening providedin an insulating layer 26. A conductive layer 25 formed using the samestep and the same material as those of the pixel electrode 21 isprovided over the insulating layer 26. The conductive layer 25 iselectrically connected to the common electrode 23 through a connector29.

Light 45 a from the backlight unit 13 is emitted to the outside of thedisplay device 10A through the substrate 12, the insulating layer 26,the pixel electrode 21, the liquid crystal layer 22, the commonelectrode 23, and the substrate 11. As materials of these layers thattransmit the light 45 a, visible-light-transmitting materials are used.

Light 45 b from the backlight unit 13 is emitted to the outside of thedisplay device 10A through the substrate 12, the transistor 14, thepixel electrode 21, the liquid crystal layer 22, the common electrode23, and the substrate 11. In this embodiment, the transistor 14electrically connected to the liquid crystal element 15 has a structurehaving a region which transmits visible light. Accordingly, a regionprovided with the transistor 14 can also be used as a display region.Thus, the aperture ratio of the pixel can be increased. As the apertureratio becomes higher, the light extraction efficiency can be increased.Therefore, the power consumption of the display device can be reduced.Moreover, the display device can have high definition.

Light 45 c from the backlight unit 13 is emitted to the outside of thedisplay device 10A through the substrate 12, the capacitor 16, the pixelelectrode 21, the liquid crystal layer 22, the common electrode 23, andthe substrate 11. In this embodiment, the capacitor 16 has a structurehaving a region which transmits visible light. Accordingly, a regionprovided with the capacitor 16 can also be used as a display region.Thus, the aperture ratio of the pixel can be increased. As the apertureratio becomes higher, the light extraction efficiency can be increased.Therefore, the power consumption of the display device can be reduced.Moreover, the display device can have high definition.

A display device 10B illustrated in FIG. 1B differs from the displaydevice 10A in that the backlight unit 13 is provided on the substrate 11side. The other structures are similar to those of the display device10A, and therefore, a description thereof is omitted.

In the display device 10A, the light 45 b first enters thevisible-light-transmitting region of the transistor 14. Then, the light45 b which has passed through the region enters the liquid crystalelement 15. In contrast, in the display device 10B, the light 45 b firstenters the liquid crystal element 15. Then, the light 45 b which haspassed through the liquid crystal element 15 enters the region whichtransmits visible light in the transistor 14. As described above, thelight from the backlight unit 13 may first enter either the transistor14 or the liquid crystal element 15.

In the display device 10A, the light 45 c enters the capacitor 16 first.Then, the light 45 c which has passed through the region enters theliquid crystal element 15. In contrast, in the display device 10B, thelight 45 c first enters the liquid crystal element 15. Then, the light45 c which has passed through the liquid crystal element 15 enters theregion which transmits visible light in the capacitor 16. As describedabove, the light from the backlight unit 13 may first enter either thecapacitor 16 or the liquid crystal element 15.

A display device of one embodiment of the present invention includes aliquid crystal element, a transistor, a capacitor, and a touch sensor.The liquid crystal element includes a pixel electrode, a liquid crystallayer, and a common electrode. The transistor is electrically connectedto the pixel electrode and the capacitor. The touch sensor is locatedcloser to a display surface than the liquid crystal element and thetransistor are. The pixel electrode, the common electrode, and thecapacitor each have a function of transmitting visible light. Thevisible light passes through the capacitor and the liquid crystalelement and is emitted to the outside of the display device.

The display device of one embodiment of the present invention can beapplied to a display device in which a touch sensor is implemented; sucha display device is also referred to as an input/output device or atouch panel.

A display device 15A illustrated in FIG. 2A has a structure in which atouch sensor unit 31 is provided on the substrate 11 side of the displaydevice 10A.

A display device 15B illustrated in FIG. 2B has a structure in which thetouch sensor unit 31 and an insulating layer 32 are provided between thesubstrate 11 and the common electrode 23 of the display device 10A. Inaddition, the display device 15B includes a conductive layer 27 and aconductive layer 28.

The conductive layer 27 formed using the same process and the samematerial as those of the pixel electrode 21 is provided over theinsulating layer 26. The conductive layer 28 formed using the sameprocess and the same material as those of the common electrode 23 isprovided in contact with the insulating layer 32. The conductive layer28 is electrically connected to the touch sensor unit 31. The conductivelayer 28 is electrically connected to the conductive layer 27 throughthe connector 29. Thus, both a signal for driving the liquid crystalelement 15 and a signal for driving the touch sensor unit 31 can besupplied through one or more FPCs connected to the substrate 12 side.There is no need to connect the FPC and the like to the substrate 11side, and thus the structure of the display device can be moresimplified. The display device in FIG. 2B can be incorporated into anelectronic device more easily than a display device in which FPCs areconnected to both of the substrate 11 side and the substrate 12 side.Furthermore, the number of components can be reduced.

In the display device 15B, the touch sensor unit 31 can be providedbetween the pair of substrates, so that the number of substrates can bereduced, and the display device can be reduced in weight and thickness.

A display device 15C illustrated in FIG. 2C has a structure in which thetouch sensor unit 31 and the insulating layer 32 are provided betweenthe substrate 12 and the insulating layer 26 of the display device 10B.In addition, the display device 15C includes a conductive layer 33.

The conductive layer 33 formed using the same process and the samematerial as one or more conductive layers included in the transistor 14is provided in contact with the insulating layer 32. The conductivelayer 33 is electrically connected to the touch sensor unit 31. In thedisplay device 15C, both a signal for driving the liquid crystal element15 and a signal for driving the touch sensor unit 31 can be supplied byone or more FPCs connected to the substrate 11 side. Thus, the displaydevice in FIG. 2C can easily be incorporated into an electronic deviceand allows a reduction in the number of components.

In the display device 15C, the touch sensor unit 31 can be providedbetween the pair of substrates, so that the number of substrates can bereduced, and the display device can be reduced in weight and thickness.

[Pixel]

Next, a pixel included in the display device of one embodiment of thepresent invention is described with reference to FIGS. 3A1 to 3B.

FIG. 3A1 is a schematic top view illustrating a pixel 900. The pixel 900illustrated in FIG. 3A1 includes four subpixels. The pixel 900illustrated in FIG. 3A1 is an example of a pixel including two-by-twosubpixels. In each of the subpixels, a transmissive liquid crystalelement 40 (not illustrated in FIGS. 3A1 and 3A2), the transistor 206,the capacitor 34, and the like are provided. In the pixel 900 in FIG.3A1, two wirings 902 and two wirings 904 are provided. FIG. 3A1illustrates display regions 918 (display regions 918R, 918G, 918B, and918W) of liquid crystal elements included in the subpixels.

The pixel 900 includes the wirings 902 and 904, and the like. Thewirings 902 function as scan lines, for example. The wirings 904function as signal lines, for example. The wirings 902 and 904 intersectwith each other at a portion.

The transistor 206 functions as a selection transistor. A gate electrodeof the transistor 206 is electrically connected to the wiring 902. Oneof a source electrode and a drain electrode of the transistor 206 iselectrically connected to the wiring 904, and the other of the sourceelectrode and the drain electrode of the transistor 206 is electricallyconnected to the liquid crystal element 40 and the capacitor 34.

Here, the wirings 902 and 904 have a light-blocking property. It ispreferable to use a light-transmitting film for layers other than thewirings 902 and 904, i.e., the transistor 206 and layers forming awiring connected to the transistor 206, the capacitor 34, and the like.FIG. 3A2 distinctively illustrates a visible-light-transmitting region900 t and a visible-light-blocking region 900 s included in the pixel900 in FIG. 3A1. When the transistor is formed using alight-transmitting film as described above, a portion other than theportion provided with the wirings can be the transmissive region 900 t.The transmissive region of the liquid crystal element can overlap withthe transistor, the wirings connected to the transistor, the capacitor,and the like, so that the aperture ratio of the pixel can be increased.

Note that as the proportion of the area of the transmissive region withrespect to the area of the pixel becomes higher, the amount oftransmitted light can be increased. The proportion of the area of thetransmissive region with respect to the area of the pixel can be higherthan or equal to 1% and lower than or equal to 95%, preferably higherthan or equal to 10% and lower than or equal to 90%, further preferablyhigher than or equal to 20% and lower than or equal to 80%, for example.In particular, the proportion is preferably higher than or equal to 40%or higher than or equal to 50%, further preferably higher than or equalto 60% and lower than or equal to 80%.

FIG. 3B and FIG. 4 are each a cross-sectional view taken alongdashed-dotted line A-B in FIG. 3A2. Note that cross sections of theliquid crystal element 40, a coloring layer 131, a light-blocking layer132, a driver circuit portion 64, and the like, which are notillustrated in the top views, are also illustrated in FIG. 3B and FIG. 4. The driver circuit portion 64 can be used as a scan line drivercircuit portion or a signal line driver circuit portion. The drivercircuit portion 64 includes a transistor 201.

It is preferable that the transistor 206 be a channel-protectivetransistor. In that case, an insulating layer 261 which functions as achannel protective layer is provided in contact with a semiconductorlayer 231 which includes a channel region. An opening portion isprovided in the insulating layer 261. The semiconductor layer 231 and aconductive layer 222 which functions as one of a source and a drain ofthe transistor 206 are electrically connected to each other via theopening portion. In addition, another opening portion is provided in theinsulating layer 261. The semiconductor layer 231 and a conductive layer232 which functions as the other of the source and the drain of thetransistor 206 are electrically connected to each other via the openingportion. Note that in the semiconductor layer 231, the resistance of aconnection portion with the conductive layer 222 and the resistance of aconnection portion with the conductive layer 232 are preferably reduced.

The capacitor 34 includes a conductive layer 262, the insulating layer261, and the conductive layer 232. The conductive layer 262 functions asa first electrode of the capacitor 34. The insulating layer 261functions as a dielectric layer of the capacitor 34. The conductivelayer 262 functions as a second electrode of the capacitor 34. That is,the conductive layer 232 functions as the other of the source and thedrain of the transistor 206 and as the second electrode of the capacitor34. The insulating layer 261 functions as the channel protective layerfor the transistor 206 and as the dielectric of the capacitor 34. Notethat the first electrode and the second electrode of the capacitor 34can serve as, for example, a lower electrode and an upper electrode,respectively.

The conductive layer 262 and the semiconductor layer 231 can be formedin the same layer. Specifically, a semiconductor layer is formed and isprocessed by a lithography method or the like, and the resistance of thesemiconductor layer which is formed in the capacitor 34 is reduced,whereby the conductive layer 262 can be formed. In that case, theconductive layer 262 has a composition similar to that of thesemiconductor layer 231. Examples of a lithography method include amethod in which a resist mask is formed over a thin film to beprocessed, the thin film is processed by etching or the like, and theresist mask is removed, and a method in which a photosensitive thin filmis formed and exposed to light and developed to be processed into adesired shape.

In this manner, when the transistor in the display device of oneembodiment of the present invention is a channel-protective transistor,the channel region of the semiconductor layer can be prevented frombeing damaged by etching at the time of formation of the source and thedrain of the transistor, for example. Accordingly, the electricalcharacteristics of the transistor can be stabilized to achieve highreliability of the transistor.

In the display device of one embodiment of the present invention, thefirst electrode of the capacitor can be formed in the same process asthat of the semiconductor layer of the transistor. The dielectric layerof the capacitor can be formed in the same process as that of thechannel protective layer of the transistor. The second electrode of thecapacitor can be formed in the same process as that of the source or thedrain of the transistor. Thus, the manufacturing process of the displaydevice of one embodiment of the present invention can be simplified, sothat the manufacturing costs can be reduced.

As illustrated in each of FIG. 3B and FIG. 4 , light is emitted from thebacklight unit 13 in a direction shown by an arrow of a broken line. Thelight from the backlight unit 13 is extracted to the outside through thetransistor 206, the capacitor 34, or the like. Accordingly, filmsforming the transistor 206 and the capacitor 34, and the like alsopreferably have a light-transmitting property. As the area of thelight-transmitting region included in the transistor 206, the capacitor34, and the like becomes larger, light from the backlight unit 13 can beused more efficiently.

As illustrated in each of FIG. 3B and FIG. 4 , the light from thebacklight unit 13 may be extracted to the outside through the coloringlayer 131. When the light is extracted through the coloring layer 131,light having a desired color can be obtained. The color of the coloringlayer 131 can be selected from red (R), green (G), blue (B), cyan (C),magenta (M), yellow (Y), and the like.

In FIG. 3B, the light from the backlight unit 13 first enters thetransistor 206, the capacitor 34, or the like. Then, the light which haspassed through the transistor 206, the capacitor 34, or the like entersthe liquid crystal element 40. After that, the light which has passedthrough the liquid crystal element 40 is extracted to the outsidethrough the coloring layer 131.

In FIG. 4 , light from the backlight unit 13 first enters the coloringlayer 131. Then, the light which has passed through the coloring layer131 enters the liquid crystal element 40. After that, the light whichhas passed through the liquid crystal element 40 is extracted to theoutside through the transistor 206, the capacitor 34, or the like.

For the transistor, the wiring, the capacitor, and the like illustratedin FIGS. 3A1 to 3B, the following materials can be used. Note that thesematerials can also be used for the visible-light-transmittingsemiconductor layer and the visible-light-transmitting conductive layerin each structure example described in this embodiment.

The semiconductor layer included in the transistor can be formed using alight-transmitting semiconductor material. As a light-transmittingsemiconductor material, a metal oxide, an oxide semiconductor, and thelike can be given. Note that an oxide semiconductor preferably containsat least indium. In particular, indium and zinc are preferablycontained. In addition, one or more kinds of aluminum, gallium, yttrium,copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel,germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium,tantalum, tungsten, magnesium, and the like may be contained.

The conductive layer included in the transistor and the capacitor can beformed using a light-transmitting conductive material. Alight-transmitting conductive material preferably includes one or morekinds selected from indium, zinc, and tin. Specifically, an In oxide, anIn—Sn oxide (also referred to as an indium tin oxide or ITO), an In—Znoxide, an In—W oxide, an In—W—Zn oxide, an In—Ti oxide, an In—Sn—Tioxide, an In—Sn—Si oxide, a Zn oxide, a Ga—Zn oxide, or the like can beused.

The conductive layer included in the transistor and the capacitor may bean oxide semiconductor that includes an impurity element and has reducedresistance. The oxide semiconductor with the reduced resistance can beregarded as an oxide conductor (OC).

For example, to form an oxide conductor, oxygen vacancies are formed inan oxide semiconductor and then hydrogen is added to the oxygenvacancies, so that a donor level is formed in the vicinity of theconduction band. The oxide semiconductor having the donor level has anincreased conductivity and becomes a conductor.

An oxide semiconductor has a large energy gap (e.g., an energy gap of2.5 eV or larger), and thus has a visible light transmitting property.Moreover, as described above, an oxide conductor is an oxidesemiconductor having a donor level in the vicinity of the conductionband. Therefore, the influence of absorption due to the donor level issmall in an oxide conductor, and an oxide conductor has a visible lighttransmitting property comparable to that of an oxide semiconductor.

The oxide conductor preferably includes one or more kinds of metalelements included in the semiconductor layer of the transistor. When twoor more layers included in the transistor are formed using the oxidesemiconductors including the same metal element, the same manufacturingapparatus (e.g., deposition apparatus or processing apparatus) can beused in two or more steps and manufacturing cost can thus be reduced.

The structure of the pixel in the display device described in thisembodiment enables efficient use of light emitted from the backlightunit. Thus, the excellent display device with reduced power consumptioncan be provided.

<2. Structure Example 2 of Display Device>

Next, a display device of one embodiment of the present invention isdescribed with reference to FIG. 5 , FIG. 6 , and FIG. 7 . FIG. 5 is aperspective view illustrating a display device 100. FIG. 6 is across-sectional view illustrating the display device 100. FIG. 5illustrates a substrate 61 with a dotted line.

The display device 100 includes a display portion 62 and the drivercircuit portion 64. An FPC 72 and an IC 73 are implemented on thedisplay device 100.

The display portion 62 includes a plurality of pixels 900 and has afunction of displaying images.

The pixel 900 includes a plurality of sub-pixels. For example, thedisplay portion 62 can display a full-color image by having one pixel becomposed of three subpixels: a subpixel exhibiting a red color, asubpixel exhibiting a green color, and a subpixel exhibiting a bluecolor. Note that the color exhibited by subpixels is not limited to red,green, and blue. A pixel may be composed of subpixels that exhibitcolors of white, yellow, magenta, or cyan, for example. In thisspecification and the like, a subpixel may be simply described as apixel.

The display device 100 may include one or both of a scan line drivercircuit and a signal line driver circuit. The display device 100 mayinclude none of the scan line driver circuit and the signal line drivercircuit. When the display device 100 includes a sensor such as a touchsensor, the display device 100 may include a sensor driver circuit. Inthis embodiment, the driver circuit portion 64 is exemplified asincluding the scan line driver circuit. The scan line driver circuit hasa function of outputting scan signals to the scan lines included in thedisplay portion 62.

In the display device 100, the IC 73 is mounted on a substrate 51 by aCOG method or the like. The IC 73 includes, for example, any one or moreof a signal line driver circuit, a scan line driver circuit, and asensor driver circuit.

The FPC 72 is electrically connected to the display device 100. The IC73 and the driver circuit portion 64 are supplied with signals or powerfrom the outside through the FPC 72. Furthermore, signals can be outputto the outside from the IC 73 through the FPC 72.

An IC may be mounted on the FPC 72. For example, an IC including any oneor more of a signal line driver circuit, a scan line driver circuit, anda sensor driver circuit may be mounted on the FPC 72.

A wiring 65 supplies signals and power to the display portion 62 and thedriver circuit portion 64. The signals and power are input to the wiring65 from the outside through the FPC 72, or from the IC 73.

FIG. 6 is a cross-sectional view including the pixel 900 and the drivercircuit portion 64. As illustrated in FIG. 6 , the display device 100includes the substrate 51, the transistor 201, the transistor 206, theliquid crystal element 40, an alignment film 133 a, an alignment film133 b, a connection portion 204, an adhesive layer 141, the coloringlayer 131, the light-blocking layer 132, an overcoat 121, the substrate61, and the like.

The liquid crystal element 40 includes a pixel electrode 111, a commonelectrode 112, and a liquid crystal layer 113. The alignment of theliquid crystal layer 113 can be controlled with the electric fieldgenerated between the pixel electrode 111 and the common electrode 112.The liquid crystal layer 113 is positioned between the alignment films133 a and 133 b.

In FIG. 6 , the pixel electrode 111 is electrically connected to theconductive layer 232. As described above, the conductive layer 232functions as the other of the source electrode and the drain electrodeof the transistor 206 and as the second electrode of the capacitor 34,and is formed using a material which transmits visible light. Theconductive layer 262 that functions as the first electrode of thecapacitor 34 is also formed using a material which transmits visiblelight. Therefore, a region where the capacitor 34 is provided can serveas the display region 918. Thus, the aperture ratio of the pixel 900 canbe increased, and the power consumption of the display device 100 can bereduced.

In FIG. 6 , alignment films are provided in contact with the liquidcrystal layer 113. The alignment films can control the alignment of theliquid crystal layer 113. In FIG. 6 , the alignment film 133 a isprovided in contact with the pixel electrode 111, and the alignment film133 b is provided in contact with the common electrode 112. Note thatthe alignment film 133 a and/or the alignment film 133 b are/is notnecessarily provided.

The liquid crystal material is classified into a positive liquid crystalmaterial with a positive dielectric anisotropy (Δε) and a negativeliquid crystal material with a negative dielectric anisotropy. Both ofthe materials can be used in one embodiment of the present invention,and an optimal liquid crystal material can be selected according to theemployed mode and design.

In one embodiment of the present invention, a negative liquid crystalmaterial is preferably used. The negative liquid crystal is lessaffected by a flexoelectric effect, which is attributed to thepolarization of liquid crystal molecules, and thus the polarity makeslittle difference in transmittance. This prevents flickering from beingrecognized by the user of the display device. The flexoelectric effectis a phenomenon in which polarization is induced by the distortion oforientation, and mainly depends on the shape of a molecule. The negativeliquid crystal material is less likely to experience the deformationsuch as spreading and bending.

Note that a liquid crystal element using any of a variety of modes canbe used as the liquid crystal element 40. For example, a liquid crystalelement using a fringe field switching (FFS) mode, a vertical alignment(VA) mode, a twisted nematic (TN) mode, an in-plane switching (IPS)mode, an axially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, anelectrically controlled birefringence (ECB) mode, a VA-IPS mode, or aguest-host mode can be used.

Furthermore, the display device 100 may be a normally black liquidcrystal display device, for example, a transmissive liquid crystaldisplay device using a vertical alignment (VA) mode. Examples of thevertical alignment mode include a multi-domain vertical alignment (MVA)mode, a patterned vertical alignment (PVA) mode, and an advanced superview (ASV) mode.

The liquid crystal element is an element that controls transmission andnon-transmission of light by optical modulation action of the liquidcrystal. The optical modulation action of liquid crystal is controlledby an electric field applied to the liquid crystal (including ahorizontal electric field, a vertical electric field, and an obliqueelectric field). For example, when the optical modulation action ofliquid crystal is controlled by an horizontal electric field, thecontrol mode can be called a horizontal electric field mode. As theliquid crystal used for the liquid crystal element, thermotropic liquidcrystal, low-molecular liquid crystal, high-molecular liquid crystal,polymer dispersed liquid crystal (PDLC), ferroelectric liquid crystal,anti-ferroelectric liquid crystal, or the like can be used. Such aliquid crystal material exhibits a cholesteric phase, a smectic phase, acubic phase, a chiral nematic phase, an isotropic phase, or the likedepending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which 5 wt. % or more of a chiralmaterial is mixed is used for the liquid crystal layer 113 in order toimprove the temperature range. The liquid crystal composition thatincludes a liquid crystal exhibiting a blue phase and a chiral materialhas a short response time and exhibits optical isotropy, which makes thealignment process unnecessary. In addition, the liquid crystalcomposition that includes a liquid crystal exhibiting a blue phase and achiral material has little viewing angle dependence. In addition, sincean alignment film does not need to be provided and rubbing treatment isunnecessary, electrostatic discharge damage caused by the rubbingtreatment can be prevented and defects or damage of the liquid crystaldisplay device in the manufacturing process can be reduced.

As the display device 100 is a transmissive liquid crystal displaydevice, a conductive material that transmits visible light is used forone or both of the pixel electrode 111 and the common electrode 112. Inaddition, a conductive material which transmits visible light is usedfor one or both of the conductive layers 232 and 262.

For example, a material containing one or more of indium (In), zinc(Zn), and tin (Sn) is preferably used for the visible-light-transmittingconductive material. Specifically, indium oxide, indium tin oxide (ITO),indium zinc oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide containingsilicon oxide (ITSO), zinc oxide, and zinc oxide containing gallium aregiven, for example. Note that a film including graphene can be used aswell. The film including graphene can be formed, for example, byreducing a film containing graphene oxide.

An oxide conductor is preferably used for one or more of the conductivelayer 232, the conductive layer 262, the pixel electrode 111, and thecommon electrode 112. The oxide conductor preferably includes one ormore metal elements that are included in the semiconductor layer 231 ofthe transistor 206. For example, the conductive layer 262 preferablycontains indium and is further preferably the In-M-Zn oxide (M is A1,Ti, Ga, Ge, Y, Zr, La, Ce, Sn, Mg, Nd, or Hf) film. The conductive layer232 is preferably an In—Zn oxide film. When the conductive layer 232 isan In—Zn oxide film, an increase in the resistance of the conductivelayer 232 which is caused in the manufacturing process of the displaydevice 100 can be prevented. Details thereof will be described later.

One or more of the conductive layer 232, the conductive layer 262, thepixel electrode 111, and the common electrode 112 may be formed using anoxide semiconductor. When two or more layers constituting the displaydevice are formed using oxide semiconductors containing the same metalelement, the same manufacturing equipment (e.g., film-formationequipment or processing equipment) can be used in two or more steps;manufacturing cost can thus be reduced.

An oxide semiconductor is a semiconductor material whose resistance canbe controlled by oxygen vacancies in the film of the semiconductormaterial and/or the concentration of impurities such as hydrogen orwater in the film of the semiconductor material. Thus, the resistivityof the oxide conductive can be controlled by performing treatment forincreasing oxygen vacancies and/or impurity concentration on the oxidesemiconductor, or treatment for reducing oxygen vacancies and/orimpurity concentration on the oxide semiconductor. Treatment forincreasing oxygen vacancies and/or the impurity concentration are/isperformed on the oxide semiconductor, whereby an oxide conductor whoseresistance is lowered as compared to the oxide semiconductor can beformed.

Note that such an oxide conductor formed using an oxide semiconductorcan be referred to as an oxide semiconductor having a high carrierdensity and a low resistance, an oxide semiconductor havingconductivity, or an oxide semiconductor having high conductivity.

In addition, the manufacturing costs can be reduced by forming the oxidesemiconductor and the oxide conductor using the same metal element. Forexample, the manufacturing costs can be reduced by using a metal oxidetarget with the same metal composition. By using the metal oxide targetwith the same metal composition, an etching gas or an etchant used inthe processing of the oxide semiconductor can also be used forprocessing of the oxide conductor. Note that even when the oxidesemiconductor and the oxide conductor have the same metal elements, thecompositions of the metal elements are different in some cases. Forexample, metal elements in the film can desorb during the fabricationprocess of the display device, which results in a different metalcomposition.

The transistor 206 includes a gate electrode 221, an insulating layer211, the semiconductor layer 231, the insulating layer 261, theconductive layer 222, and the conductive layer 232. For example, thesemiconductor layer 231 preferably includes indium and is furtherpreferably an In-M-Zn oxide (M is Al, Ti, Ga, Ge, Y, Zr, La, Ce, Nd, Sn,or Hf) film.

The gate electrode 221 and the semiconductor layer 231 overlap with theinsulating layer 211 positioned therebetween. Specifically, the gateelectrode 221 has a region which overlaps with the channel region of thesemiconductor layer 231 with the insulating layer 211 positionedtherebetween.

The semiconductor layer 231 is provided in contact with the insulatinglayer 211. The insulating layer 261 is provided in contact with thesemiconductor layer 231 and the insulating layer 211. The conductivelayer 222 is electrically connected to the semiconductor layer 231 viaan opening portion provided in the insulating layer 261. The conductivelayer 232 is electrically connected to the semiconductor layer 231 viaan opening portion provided in the insulating layer 261.

The insulating layer 211 functions as a gate insulating layer of thetransistor 206. The semiconductor layer 231 includes the channel region.The insulating layer 261 functions as a channel protective layer of thetransistor 206. The conductive layer 222 function as one of the sourceand drain electrodes of the transistor 206. The conductive layer 232functions as the other of the source electrode and the drain electrodeof the transistor 206.

The gate electrode 221 can serve as part of a scan line. In other words,the scan line can have a portion which overlaps with the channel regionof the transistor 206. The conductive layer 222 can serve as part of asignal line. Accordingly, the electrical resistances of the gateelectrode 221 and the conductive layer 222 are preferably sufficientlylow. Therefore, the gate electrode 221 and the conductive layer 222 arepreferably formed using metal, an alloy, or the like. A material whichblocks visible light may be used for the gate electrode 221 and theconductive layer 222.

As described above, a material having a function of transmitting visiblelight is preferably used for the conductive layer 232. That is, theconductive layer 222 and the conductive layer 232 in the transistor 206can be formed using different materials in one embodiment of the presentinvention.

Specifically, a conductive material which transmits visible light andcan be used for the conductive layer 232 or the like can have largerresistivity than a conductive material which blocks visible light, suchas copper or aluminum. Therefore, to prevent signal delay, bus linessuch as scan lines and signal lines are preferably formed using aconductive material which has low resistivity and blocks visible light,e.g., a metal material such as copper. That is, the gate electrode 221and the conductive layer 222 are preferably formed using a metalmaterial such as copper. Note that a conductive material which transmitsvisible light can be used for bus lines depending on the size of pixels,the widths and thicknesses of the bus lines, or the like.

The use of a conductive layer blocking visible light for the gateelectrode 221 can prevent irradiation of the semiconductor layer 231with light from the backlight, which can reduce variations incharacteristics of the transistor and increase the reliability.

Since the light-blocking layer 132 is provided on the substrate 61 sideof the semiconductor layer 231 and the gate electrode 221 blockingvisible light is provided on the substrate 51 side of the semiconductorlayer 231, the semiconductor layer 231 can be prevented from beingirradiated with external light and light from the backlight.

In one embodiment of the present invention, the conductive layerblocking visible light may overlap with part of the semiconductor layer231 and does not necessarily overlap with the other part of thesemiconductor layer 231. For example, the conductive layer blockingvisible light may overlap with at least the channel region of thesemiconductor layer 231.

The transistor 206 is covered with the insulating layers 212, 214, and215. Note that insulating layers 212, 214, and 215 can be considered asthe component of the transistor 206. The insulating layer 215 functionsas a planarization layer.

The insulating layer 212 preferably contains oxygen at a higherproportion than the stoichiometric composition. In addition, theinsulating layer 214 and the insulating layer 215 may each containoxygen at a higher proportion than the stoichiometric composition. Partof oxygen contained in the insulating layer 212 and the like passesthrough the insulating layer 261 and is supplied to the channel regionof the semiconductor layer 231 by heat treatment or the like.Accordingly, in the case where the channel region of the semiconductorlayer 231 is an oxide semiconductor, oxygen vacancies of the channelregion can be reduced. Therefore, the resistance of the channel regionof the semiconductor layer 231 can be increased, variation in theelectrical characteristics of the transistor can be prevented, and thereliability can be improved.

For example, in the case where an oxide conductor is used for theconductive layer 262, oxygen contained in the insulating layer 212 andthe like diffuses into the conductive layer 262, so that the resistanceof the conductive layer 262 might be increased. However, the conductivelayer 232 is formed using a material which is less likely to bepermeable to oxygen. Accordingly, the oxygen contained in the insulatinglayer 212 does not easily supplied to the conductive layer 262.Therefore, even in the case where an oxide conductor is used for theconductive layer 262, the increase in the resistance of the conductivelayer 262 can be inhibited.

In the display device 100, the coloring layer 131 and the light-blockinglayer 132 are provided closer to the substrate 61 than liquid crystallayer 113 is. The coloring layer 131 is positioned in a region thatoverlaps with at least the display region 918 of the pixel 900. Thelight-blocking layer 132 is provided in a portion other than the displayregion 918 (a non-display region). The light-blocking layer 132 overlapswith at least a part of the transistor 206.

The overcoat 121 is preferably provided between the liquid crystal layer113 and each of the coloring layer 131 and the light-blocking layer 132.The overcoat 121 can reduce the diffusion of an impurity contained inthe coloring layer 131 and the light-blocking layer 132 and the likeinto the liquid crystal layer 113.

The substrates 51 and 61 are bonded to each other with the adhesionlayer 141. The liquid crystal layer 113 is encapsulated in a region thatis surrounded by the substrates 51 and 61, and the adhesive layer 141.

When the display device 100 functions as a transmissive liquid crystaldisplay device, two polarizing plates are positioned in a way that thepixel 900 is sandwiched between the two polarizing plates. FIG. 6illustrates the polarizing plate 130 on the substrate 61 side. Lightfrom a backlight provided on the outside of the polarizing plate on thesubstrate 51 side enters the display device 100 through the polarizingplate. In this case, the optical modulation of the light can becontrolled by controlling the alignment of the liquid crystal layer 113with a voltage supplied between the pixel electrode 111 and the commonelectrode 112. That is, the intensity of light emitted through thepolarizing plate 130 can be controlled. Furthermore, the coloring layer131 absorbs light of wavelengths other than a specific wavelength rangefrom the incident light. As a result, the ejected light is light thatexhibits red, blue, or green colors, for example.

In addition to the polarizing plate, a circularly polarizing plate canbe used, for example. An example of a circularly polarizing plate is astack including a linear polarizing plate and a quarter-wave retardationplate. The circular polarizing plate can reduce the viewing angledependence of the display quality of the display device.

The liquid crystal element 40 is preferably driven in a guest-hostliquid crystal mode. When the guest-host liquid crystal mode is used, apolarizing plate does not need to be used. Since absorption of light bya polarizing plate can be eliminated, light extraction efficiency can beincreased, and bright display of the display device can be achieved.

The driver circuit portion 64 includes the transistor 201.

The transistor 201 includes the gate electrode 221, the insulating layer211, the semiconductor layer 231, the insulating layer 261, a conductivelayer 224, and a conductive layer 225. One of the conductive layers 224and 225 functions as a source electrode, and the other thereof functionsas a drain electrode. An opening portion is provided in the insulatinglayer 261. The conductive layer 224 and the semiconductor layer 231 areelectrically connected to each other via the opening portion. Anotheropening portion is provided in the insulating layer 261. The conductivelayer 225 and the semiconductor layer 231 are electrically connected toeach other via the opening portion. Note that in the semiconductor layer231, the resistance of a connection portion with the conductive layer224 and the resistance of a connection portion with the conductive layer225 are preferably reduced.

The transistor provided in the driver circuit portion 64 does not needto have a function of transmitting visible light. In other words, theconductive layers 224 and 225 can be formed using a metal material orthe like. Specifically, the same material as that of conductive layer222 can be used. In addition, the conductive layer 224, the conductivelayer 225, and the conductive layer 222 can be formed in the sameprocess.

In the connection portion 204, the wiring 65 and a conductive layer 251are connected to each other, and the conductive layer 251 and aconnector 242 are connected to each other. That is, in the connectionportion 204, the conductive layer 225 is electrically connected to theFPC 72 through the conductive layer 251 and the connector 242. Byemploying this configuration, signals and power can be supplied from theFPC 72 to the wiring 65.

The wiring 65 can be formed using the same material and the same processas those used for the conductors 224 and 225 of the transistor 201 andthe conductive layer 222 of the transistor 206. The conductive layer 251can be formed using the same material and the same process as those usedfor the pixel electrode 111 included in the liquid crystal element 40.Forming the conductive layers included in the connection portion 204 insuch a manner, i.e., using the same materials and the same processes asthose used for the conductive layers used in the pixel 900 and thedriver circuit portion 64, is preferable because an increase in thenumber of steps can be prevented.

The transistors 201 and 206 may have the same structure or differentstructures. That is, the transistor included in the driver circuitportion 64 and the transistor included in the pixel 900 may have thesame structure or different structures. For example, the transistorincluded in the driver circuit portion 64 is not necessarily achannel-protective transistor, and may be a channel-etched transistor.In addition, the driver circuit portion 64 may have a plurality oftransistors with different structures, and the pixel 900 may have aplurality of transistors with different structures. For example, thedriver circuit portion 64 may have a channel-protective transistor and achannel-etched transistor. In addition, the pixel 900 may have achannel-protective transistor and a channel-etched transistor.

The semiconductor layer 231 of the transistor 201 and the semiconductorlayer 231 of the transistor 206 may be formed using different materials.For example, amorphous silicon, low-temperature polycrystalline silicon,or the like may be used for the semiconductor layer 231 of thetransistor 201, and an oxide semiconductor may be used for thesemiconductor layer 231 of the transistor 206. When amorphous silicon,low-temperature polycrystalline silicon, or the like is used for thesemiconductor layers of some transistors, the on-state current of thetransistors can be increased. Consequently, a circuit capable ofhigh-speed operation can be obtained. Furthermore, the area occupied bya circuit portion can be reduced. The use of the transistor having ahigh on-state current can reduce signal delay in wirings and cansuppress display unevenness even in a display panel or a display devicein which the number of wirings is increased because of an increase insize or resolution. Moreover, with such a structure, a highly reliabletransistor can be formed.

FIG. 7 illustrates a modification example of the display device 100. Thestructure of the display device 100 illustrated in FIG. 7 is differentfrom that of the display device 100 illustrated in FIG. 6 in that thetransistor 206 and the transistor 201 each have a gate electrode 223.

The gate electrode 223 is provided in contact with the insulating layer214. The gate electrode 223 is provided to include a region whichoverlaps with the semiconductor layer 231. A metal material such ascopper can be used for the gate electrode 223, for example.Specifically, the same material as that of the gate electrode 221 can beused, for example. Note that the gate electrode 223 may be formed usinga material different from that of the gate electrode 221. For example,one of the gate electrodes 221 and 223 may be formed using a materialhaving a light-blocking property, such as a metal material.Alternatively, one of the gate electrodes 221 and 223 may be formedusing an oxide conductor.

The gate electrode 221 and the gate electrode 223 can be electricallyconnected to each other. Transistors having such a structure in whichtwo gate electrodes are electrically connected to each other can have ahigher field-effect mobility and thus have higher on-state current thanother transistors. Consequently, a circuit capable of high-speedoperation can be obtained. Furthermore, the area occupied by a circuitportion can be reduced. The use of the transistor having a high on-statecurrent can reduce signal delay in wirings and can suppress displayunevenness even in a display panel or a display device in which thenumber of wirings is increased because of an increase in size orresolution. Moreover, with such a structure, a highly reliabletransistor can be formed.

Note that some of the transistors included in the display device 100each may have the gate electrode 223 and the other thereof each may haveno gate electrode 223. For example, the transistor 201 may have the gateelectrode 223, and the transistor 206 may have no gate electrode 223.Alternatively, for example, the transistor 206 may have the gateelectrode 223, and the transistor 201 may have no gate electrode 223.Further alternatively, some or all of the transistors included in thedisplay device 100 may each have no gate electrode 221.

Next, the details of the materials that can be used for components ofthe display device of one embodiment of the present invention and thelike are described. Note that description on the components alreadydescribed is omitted in some cases. The materials described below can beused as appropriate in the display device, the touch panel, and thecomponents thereof described later.

<<Substrates 51 and 61>>

There are no large limitations on the material of the substrate used inthe display device of one embodiment of the present invention; a varietyof substrates can be used. For example, a glass substrate, a quartzsubstrate, a sapphire substrate, a semiconductor substrate, a ceramicsubstrate, a metal substrate, or a plastic substrate can be used.

The weight and thickness of the display device can be reduced by using athin substrate. Furthermore, a flexible display device can be obtainedby using a substrate that is thin enough to have flexibility.

The display device of one embodiment of the present invention isfabricated by forming a transistor and the like over the fabricationsubstrate, then transferring the transistor and the like on anothersubstrate. The use of the fabrication substrate enables the following: aformation of a transistor with favorable characteristics; a formation ofa transistor with low power consumption; a manufacturing of a durabledisplay device; an addition of heat resistance to the display device; amanufacturing of a more lightweight display device; or a manufacturingof a thinner display device. Examples of a substrate to which atransistor is transferred include, in addition to the substrate overwhich the transistor can be formed, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), and the like),a leather substrate, a rubber substrate, and the like.

<<Semiconductorlayer>>

A semiconductor material used for the semiconductor layer is notparticularly limited, and for example, an oxide semiconductor, silicon,or germanium can be used. There is no particular limitation on thecrystallinity of a semiconductor material used for the semiconductorlayer, and an amorphous semiconductor or a semiconductor havingcrystallinity (a microcrystalline semiconductor, a polycrystallinesemiconductor, a single-crystal semiconductor, or a semiconductor partlyincluding crystal regions) may be used. The use of a semiconductorhaving crystallinity is preferable as the degradation of a transistor'scharacteristics can be reduced.

For example, a Group 14 element, a compound semiconductor, or an oxidesemiconductor can be used for the semiconductor layer. Typically, asemiconductor including silicon, a semiconductor including galliumarsenide, or an oxide semiconductor including indium can be used for thesemiconductor layer.

An oxide semiconductor is preferably used for the semiconductor in whichthe channel of a transistor is formed. In particular, using an oxidesemiconductor with a larger bandgap than that of silicon is preferable.The use of a semiconductor material with a larger bandgap than that ofsilicon and a small carrier density is preferable because the currentduring the off state of the transistor can be reduced.

The above description, the description in Embodiment 4 and the like canbe referred to for the oxide semiconductor.

The use of such an oxide semiconductor for the semiconductor layer makesit possible to provide a highly reliable transistor in which a change inthe electrical characteristics is reduced.

Charge accumulated in a capacitor through the transistor can be retainedfor a long time because of low off-state current of the transistor. Theuse of such a transistor in pixels allows a driver circuit to stop whilethe gray level of a displayed image is maintained. As a result, adisplay device with extremely low power consumption is obtained.

The transistors 201 and 206 preferably include an oxide semiconductorthat is highly purified to reduce the formation of oxygen vacancies.This makes the current during the off-state (off-state current) of thetransistor low. Accordingly, an electrical signal such as an imagesignal can be held for a long period, and a writing interval can be setlong in an on state. Thus, the frequency of refresh operation can bereduced, which leads to an effect of reducing power consumption.

In the transistors 201 and 206, a relatively high field-effect mobilitycan be obtained, whereby high-speed operation is possible. The use ofsuch transistors that are capable of high-speed operation in the displaydevice enables the formation of the transistor in the display portionand the transistors in the driver circuit portion over the samesubstrate. This means that a semiconductor device separately formed witha silicon wafer or the like does not need to be used as the drivercircuit, which enables a reduction in the number of components in thedisplay device. In addition, using the transistor that can operate athigh speed in the display portion also can enable the provision of ahigh-quality image.

<<Insulating Layer>>

An organic insulating material or an inorganic insulating material canbe used as an insulating material that can be used for the insulatinglayer, the overcoat, the spacer, or the like included in the displaydevice. Examples of an organic insulating material include an acrylicresin, an epoxy resin, a polyimide resin, a polyamide resin, apolyimide-amide resin, a siloxane resin, a benzocyclobutene-based resin,and a phenol resin. Examples of an inorganic insulating film include asilicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, an aluminum oxide film, a hafnium oxidefilm, an yttrium oxide film, a zirconium oxide film, a gallium oxidefilm, a tantalum oxide film, a magnesium oxide film, a lanthanum oxidefilm, a cerium oxide film, and a neodymium oxide film.

<<Conductive Layer>>

For the conductive layer such as the gate, the source, and the drain ofa transistor and the wiring, the electrode, and the like of the displaydevice, a single-layer structure or a layered structure using any ofmetals such as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, silver, tantalum, and tungsten, or an alloycontaining any of these metals as its main component can be used. Forexample, a two-layer structure in which a titanium film is stacked overan aluminum film; a two-layer structure in which a titanium film isstacked over a tungsten film; a two-layer structure in which a copperfilm is stacked over a molybdenum film; a two-layer structure in which acopper film is stacked over an alloy film containing molybdenum andtungsten; a two-layer structure in which a copper film is stacked overan alloy film containing copper, magnesium, and aluminum; a three-layerstructure in which titanium film or a titanium nitride film, an aluminumfilm or a copper film, and a titanium film or a titanium nitride filmare stacked in this order; a three-layer structure in which a molybdenumfilm or a molybdenum nitride film, an aluminum film or a copper film,and a molybdenum film or a molybdenum nitride film are stacked in thisorder; or the like can be employed. For example, in the case where theconductive layer has a three-layer structure, it is preferable that eachof the first and third layers be a film formed of titanium, titaniumnitride, molybdenum, tungsten, an alloy containing molybdenum andtungsten, an alloy containing molybdenum and zirconium, or molybdenumnitride, and that the second layer be a film formed of a low-resistancematerial such as copper, aluminum, gold, silver, or an alloy containingcopper and manganese. Note that light-transmitting conductive materialssuch as ITO, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or ITSOmay be used.

An oxide conductor may be formed by controlling the resistivity of theoxide semiconductor.

<<Adhesive Layer 141>>

A curable resin such as a heat-curable resin, a photocurable resin, or atwo-component type curable resin can be used for the adhesive layer 141.For example, an acrylic resin, a urethane resin, an epoxy resin, or asiloxane resin can be used.

<<Connector 242>>

As the connector 242, for example, an anisotropic conductive film (ACF)or an anisotropic conductive paste (ACP) can be used.

<<Coloring Layer 131>>

The coloring layer 131 is a colored layer that transmits light in aspecific wavelength range. Examples of materials that can be used forthe coloring layer 131 include a metal material, a resin material, and aresin material containing a pigment or dye.

<<Light-Blocking Layer 132>>

The light-blocking layer 132 is provided, for example, between adjacentcoloring layers 131 for different colors. A black matrix formed with,for example, a metal material or a resin material containing a pigmentor dye can be used as the light-blocking layer 132. Note that it ispreferable to provide the light-blocking layer 132 also in a regionother than the pixel 900, such as the driver circuit portion 64, inwhich case leakage of guided light or the like can be inhibited.

<3. Example of Fabricating Method of Display Device>

An example of a method for manufacturing the display device 100illustrated in FIG. 6 is described with reference to FIGS. 8A to 8C andFIGS. 9A and 9B. Note that in this example of the manufacturing method,a different display device or the like of this embodiment can bemanufactured by modifying the structures of transistors, a liquidcrystal element, and the like which are formed.

The thin films constituting the display device (i.e., the insulatingfilm, the semiconductor film, the conductive film, and the like) can beformed by any of a sputtering method, a chemical vapor deposition (CVD)method, a vacuum evaporation method, a pulsed laser deposition (PLD)method, an atomic layer deposition (ALD) method, and the like. Asexamples of the CVD method, a plasma-enhanced CVD (PECVD) method or athermal CVD method can be given. As an example of the thermal CVDmethod, a metal organic CVD (MOCVD) method can be given.

Alternatively, the thin films constituting the display device (i.e., theinsulating film, the semiconductor film, the conductive film, and thelike) can be formed by a method such as spin coating, dipping, spraycoating, inkjet printing, dispensing, screen printing, or offsetprinting, or with a doctor knife, a slit coater, a roll coater, acurtain coater, or a knife coater.

The thin films constituting the display device can be processed using alithography method or the like. Alternatively, island-shaped thin filmsmay be formed by a film formation method using a blocking mask.Alternatively, the thin films may be processed by a nano-imprintingmethod, a sandblasting method, a lift-off method, or the like.

In the case of processing by a photolithography method, light with ani-line (with a wavelength of 365 nm), light with a g-line (with awavelength of 436 nm), light with an h-line (with a wavelength of 405nm), and light in which the i-line, the g-line, and the h-line are mixedcan be used. Alternatively, ultraviolet light, KrF laser light, ArFlaser light, or the like can be used. Exposure may be performed byliquid immersion exposure technique. As light used in exposure, extremeultra-violet light (EUV), X-rays, or the like can be given. An electronbeam can be used instead of a light used in exposure. It is preferableto use extreme ultra-violet light, X-rays, or an electron beam becauseextremely minute processing can be performed. Note that when exposure isperformed by scanning of a beam such as an electron beam, a photomask isnot needed.

For etching of the thin film, dry etching, wet etching, a sandblastmethod, or the like can be used.

To manufacture the display device 100, first, a conductive layer isformed over the substrate 51 and processed by a lithography method orthe like, whereby the wiring 902 and the gate electrode 221 are formed.As described above, the wiring 902 functions as the scan line. Inaddition, the gate electrode 221 can serve as part of the scan line. Itis preferable to form the wiring 902 and the gate electrode 221 with theuse of a conductive material which has low resistivity and blocksvisible light, e.g., a metal material such as copper.

Next, the insulating layer 211 is formed. As described above, theinsulating layer 211 functions as the gate insulating layers of thetransistors provided in the display device 100.

After that, a semiconductor layer is formed and processed by alithography method or the like to form the semiconductor layer 231 and asemiconductor layer 262 a (FIG. 8A). The semiconductor layer 231includes regions which function as the channel regions of thetransistors provided in the display device 100. An oxide semiconductoris preferably used for the semiconductor layer 231 and the semiconductorlayer 262 a. For example, each of the semiconductor layer 231 and thesemiconductor layer 262 a preferably includes indium and is furtherpreferably an In-M-Zn oxide (M is Al, Ti, Ga, Ge, Y, Zr, La, Ce, Nd, Sn,or Hf) film.

Next, the insulating layer 261 is formed (FIG. 8B). As described above,the insulating layer 261 functions as the channel protective layers ofthe transistors provided in the display device 100 and as the dielectriclayer of the capacitor provided in the display device 100.

The insulating layer 261 can be a silicon oxynitride film, for example.The insulating layer 261 can be formed by, for example, a CVD method,specifically, a PECVD method. When a film including silicon oxynitrideis formed by a CVD method, for example, a silane gas is used as adeposition gas and a nitrogen oxide gas such as a nitrous oxide gas isused. In that case, hydrogen or the like is supplied to thesemiconductor layer 262 a during formation of the insulating layer 261,so that the resistance of the semiconductor layer 262 a can be reduced.Thus, the conductive layer 262 that functions as the first electrode ofthe capacitor provided in the display device 100 can be formed.

Note that the insulating layer 261 formed by the above method includesammonia. Even when the hydrogen or the like supplied to the conductivelayer 262 is diffused to the outside by heat treatment performed afterthe formation of the insulating layer 261, an increase in theresistivity of the conductive layer 262 can be prevented because ammoniaincluded in the insulating layer 261 is supplied to the conductive layer262.

Hydrogen or the like is supplied to the semiconductor layer 231 duringthe formation of the insulating layer 261, so that the resistance of thechannel region included in the semiconductor layer 231 might be reduced.However, the resistance of the channel region included in thesemiconductor layer 231 can be increased in a later step; therefore, thereduction in the resistance at the time of the formation of theinsulating layer 261 does not cause a problem.

Next, an opening reaching the semiconductor layer 231 is provided in theinsulating layer 261, and then a semiconductor layer 232 a is formed.Note that the semiconductor layer 232 a includes a region which overlapswith the conductive layer 262. In addition, the semiconductor layer 232a is electrically connected to the semiconductor layer 231 via theopening portion provided in the insulating layer 261.

An In—Zn oxide is preferably used for the semiconductor layer 232 a, forexample. The In—Zn oxide is preferably formed by, for example, asputtering method in an argon (argon 100%) atmosphere at a substratetemperature that is set to a temperature which is not increased byintentional heating. When an In—Zn oxide is used for the semiconductorlayer 232 a, the conductive layer 232 can be formed by a reduction inthe resistance of the semiconductor layer 232 a in a later step, and theresistance of the formed conductive layer 232 can be prevented frombeing increased in a step after the step. Note that an oxidesemiconductor other than an In—Zn oxide may be used for thesemiconductor layer 232 a. In that case, the semiconductor layer 232 apreferably includes one or more of the metal elements included in thesemiconductor layer 231. In the case where the semiconductor layer 232 aincludes one or more of the metal elements included in the semiconductorlayer 231, the same manufacturing apparatus (e.g., a depositionapparatus or a processing apparatus) can be used in two or more steps,so that the manufacturing costs can be reduced.

Next, opening portions reaching the semiconductor layer 231 are formedin the insulating layer 261, and then the conductive layer 222, theconductive layer 224, the conductive layer 225, and the wiring 65 areformed (FIG. 8C). Note that the conductive layer 222, the conductivelayer 224, and the conductive layer 225 are electrically connected tothe semiconductor layer 231 via the openings provided in the insulatinglayer 261.

As described above, the conductive layer 222 functions as one of thesource and drain electrodes of the transistor 206. The conductive layer224 functions as one of the source and drain electrodes of thetransistor 201. The conductive layer 225 functions as the other of thesource and drain electrodes of the transistor 201. The conductive layer222 can serve as part of the signal line. The conductive layer 222, theconductive layer 224, the conductive layer 225, and the wiring 65 arepreferably formed using a conductive material which has low resistanceand blocks visible light, e.g., a metal material such as copper.

Since the insulating layer 261 functions as the channel protectivelayer, the channel region of the semiconductor layer 231 can beprevented from being damaged by etching at the time of formation of theconductive layer 222, the semiconductor layer 232 a, the conductivelayer 224, and the conductive layer 225. Accordingly, the electricalcharacteristics of the transistors included in the display device 100can be stabilized, so that the transistors can have high reliability.

After the formation of the opening portions in the insulating layer 261,an inert gas such as argon may be supplied to the semiconductor layer231, whereby contact portions with the source and drain electrodes ofthe transistor 206 and/or the transistor 201 may have n-typeconductivity. A method for supplying an inert gas can be a sputteringmethod, an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, or plasma treatment, for example.Thus, the on-state current of the transistor 206 and/or the transistor201 can be increased, and the operation speed of the display device 100can be increased.

Next, the insulating layer 212 is formed. For the insulating layer 212,for example, a film including silicon oxynitride can be used. Theinsulating layer 212 can be formed by, for example, a CVD method,specifically, a PECVD method. When a film including silicon oxynitrideis formed by a CVD method, for example, a silane gas is used as adeposition gas and a nitrogen oxide gas such as a nitrous oxide gas isused. In that case, hydrogen or the like is supplied to thesemiconductor layer 232 a during formation of the insulating layer 212,so that the resistance of the semiconductor layer 232 a can be reduced.Thus, the conductive layer 232 which functions as the source or drainelectrode of the transistor 206 and as the second electrode of thecapacitor 34 can be formed.

The insulating layer 212 preferably contains oxygen at a higherproportion than the stoichiometric composition. For example, in the casewhere the insulating layer 212 is formed by a CVD method, oxygen, amixed gas of oxygen and a nitrogen oxide gas such as nitrous oxide ornitrogen dioxide, or the like is used as a deposition gas, wherebyoxygen can be contained in the insulating layer 212. Alternatively,oxygen may be introduced into the insulating layer 212 by an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, plasma treatment, or the like after the insulatinglayer 212 is formed.

Part of oxygen contained in the insulating layer 212 and the like passesthrough the insulating layer 261 and is supplied to the channel regionof the semiconductor layer 231 by heat treatment or the like.Accordingly, in the case where the channel region of the semiconductorlayer 231 is an oxide semiconductor, oxygen vacancies of the channelregion can be reduced. Therefore, the resistance of the channel regionof the semiconductor layer 231 can be increased, variation in theelectrical characteristics of the transistor can be prevented, and thereliability can be improved.

For example, in the case where an oxide conductor is used for theconductive layer 262, oxygen contained in the insulating layer 212 andthe like diffuses into the conductive layer 262, so that the resistanceof the conductive layer 262 might be increased. However, the conductivelayer 232 is formed using a material which is less likely to bepermeable to oxygen. Accordingly, the oxygen contained in the insulatinglayer 212 does not easily supplied to the conductive layer 262.Therefore, even in the case where an oxide conductor is used for theconductive layer 262, the increase in the resistance of the conductivelayer 262 can be inhibited.

Next, the insulating layer 214 is formed. After that, the insulatinglayer 215 is formed. After the insulating layer 215 is formed,planarization treatment is performed on the insulating layer 215 by achemical mechanical polishing (CMP) method or the like (FIG. 9A).

Next, an opening portion reaching the conductive layer 232 and anopening portion reaching the wiring 65 are formed in the insulatinglayer 215, the insulating layer 214, and the insulating layer 212. Then,a conductive layer is provided and processed by a lithography method orthe like, whereby the pixel electrode 111 and the conductive layer 251are formed. The pixel electrode 111 and the conductive layer 251 areelectrically connected to the conductive layer 232 and the wiring 65,respectively, via the opening portions provided in the insulating layer215, the insulating layer 214, and the insulating layer 212. Asdescribed above, a material having a function of transmitting visiblelight is used for the pixel electrode 111.

Next, the alignment film 133 a is formed to cover the pixel electrode111 (FIG. 9B). After that, the light-blocking layer 132, the coloringlayer 131, the overcoat 121, the common electrode 112, and the alignmentfilm 133 b are formed over the substrate 61 (FIG. 9C). As describedabove, a material having a function of transmitting visible light isused for the common electrode 112.

The liquid crystal layer 113 is sealed with the adhesive layer 141between the substrate 51 illustrated in FIG. 9B and the substrate 61illustrated in FIG. 9C. Then, the connector 242, the FPC 72, and thepolarizing plate 130 are formed. Through the above steps, the displaydevice 100 having the structure in FIG. 6 can be formed.

As described above, in the method for manufacturing the display deviceof one embodiment of the present invention, the semiconductor layer 231of the transistor 206 and the conductive layer 262 of the capacitor 34can be formed in the same process. In addition, the insulating layer 261which serves as the insulating layer functioning as the channelprotective layer of the transistor 206 and also serves as the insulatinglayer functioning as the dielectric layer of the capacitor 34 can beformed in one process. The conductive layer 232 which serves as theconductive layer functioning as the other of the source and drainelectrodes of the transistor 206 and also serves as the conductive layerfunctioning as the second electrode of the capacitor 34 can be formed inone process. Accordingly, the manufacturing process of the displaydevice 100 can be simplified, so that the manufacturing costs of thedisplay device 100 can be reduced.

As described above, the conductive layer 262 and the conductive layer232 each have a function of transmitting visible light. Thus, theaperture ratio of the pixel 900 of the display device 100 can beincreased. The light extraction efficiency can be increased as theaperture ratio becomes higher, so that the power consumption of thedisplay device 100 can be reduced. In addition, the resolution of animage displayed by the display device 100 can be increased.

<<4. Pixel Arrangement Example>

FIGS. 10A and 10B shows the pixel 900 and an example of an arrangementof sub-pixels included in the pixel 900. Each of FIGS. 10A and 10Billustrates an example in which a red subpixel R, a green subpixel G,and a blue subpixel B form one pixel. In FIGS. 10A and 10B, a pluralityof scan lines 81 extend in an x direction, and a plurality of signallines 82 extend in a y direction. The scan lines 81 and the signal lines82 intersect with each other.

As shown by the dashed two dotted line in FIG. 10A, a subpixel includesthe transistor 206, the capacitor 34, and the liquid crystal element 40.The gate electrode of the transistor 206 is electrically connected tothe scan line 81. One of the source electrode and the drain electrode ofthe transistor 206 is electrically connected to the signal line 82, andthe other thereof is electrically connected to the second electrode ofthe capacitor 34 and a first electrode of the liquid crystal element 40.The first electrode of the capacitor 34 and the first electrode of theliquid crystal element 40 are each supplied with a constant potential.

FIGS. 10A and 10B show examples where the source-line inversion drivingis adopted. Signals A1 and A2 are signals with the same polarity.Signals B1 and B2 are signals with the same polarity. Signals A1 and B1are signals with different polarities. Signals A2 and B2 are signalswith different polarities.

As the definition of the display device become higher, the distancebetween the subpixels become shorter. Thus, as shown in the frameoutlined in a dashed-dotted line in FIG. 10A, in the subpixel where thesignal A1 is input, the liquid crystal is easily affected by potentialsin both the signal A1 and the signal B1, in the vicinities of the signalline 82 where the signal B1 is input. This can make the liquid crystalmore prone to alignment defects.

In FIG. 10A, the direction in which a plurality of subpixels exhibitingthe same color are aligned is the y direction, and is substantiallyparallel to the direction that the signal lines 82 extend in. As shownin the frame outlined by a dashed-dotted line in FIG. 10A, subpixelsexhibiting different colors are adjacent to each other, with the longersides of the subpixels facing each other.

In FIG. 10B, the direction in which the plurality of subpixelsexhibiting the same color are aligned is the x direction, and intersectswith the direction that the signal lines 82 extend in. As shown in theframe outlined in a dashed-dotted line in FIG. 10B, subpixels exhibitingthe same color are adjacent to each other, with the shorter sides of thesubpixels facing each other.

When the side of the subpixel that is substantially parallel to thedirection in which the signal lines 82 extend is the shorter sides ofthe subpixel as illustrated in FIG. 10B, the region where the liquidcrystal is more prone to alignment defects can be made narrower,compared with the case (illustrated in FIG. 10A) where the side of thesubpixel that is substantially parallel to the direction in which thesignal lines 82 extend is the longer sides of the subpixel. When theregion where the liquid crystal is more prone to alignment defects ispositioned between subpixels exhibiting the same color as illustrated inFIG. 10B, display defects are less easily recognized by a user of thedisplay device when compared with the case (see FIG. 10A) where theregion is positioned between subpixels exhibiting different colors. Inone embodiment of the present invention, a direction in which thesubpixels exhibiting the same color are arranged preferably intersectswith the direction in which the signal line 82 extends.

<5. Structure Example 3 of Display Device>

One embodiment of the present invention can be applied to a displaydevice in which a touch sensor is implemented; such a display device isalso referred to as an input/output device or a touch panel. Any of thestructures of the display device described above can be applied to thetouch panel. In this embodiment, the description focuses on an examplein which the touch sensor is implemented in the display device 100.

There is no limitation on the sensing element (also referred to as asensor element) included in the touch panel of one embodiment of thepresent invention. A variety of sensors capable of sensing an approachor a contact of an object such as a finger or a stylus can be used asthe sensor element.

For example, a variety of types such as a capacitive type, a resistivetype, a surface acoustic wave type, an infrared type, an optical type,and a pressure-sensitive type can be used for the sensor.

In this embodiment, a touch panel including a capacitive sensor elementis described as an example.

Examples of the capacitive touch sensor element include a surfacecapacitive touch sensor element and a projected capacitive touch sensorelement. Examples of the projected capacitive sensor element include aself-capacitive sensor element and a mutual capacitive sensor element.The use of a mutual capacitive sensor element is preferable becausemultiple points can be sensed simultaneously.

The touch panel of one embodiment of the present invention can have anyof a variety of structures, including a structure in which a displaydevice and a sensor element that are separately formed are attached toeach other and a structure in which an electrode and the like includedin a sensor element are provided on one or both of a substratesupporting a display element and a counter substrate.

FIGS. 11A and 11B and FIG. 12 each illustrate an example of the touchpanel. FIG. 11A is a perspective view of a touch panel 350. FIG. 11B isa developed view of the schematic perspective view of FIG. 11A. Notethat for simplicity, FIGS. 11A and 11B illustrate only the majorcomponents. In FIG. 11B, the outlines of the substrate 61 and asubstrate 162 are illustrated only in dashed lines. FIG. 12 is across-sectional view of the touch panel 350.

The touch panel 350 has a structure in which a display device and asensor element that are fabricated separately are bonded together.

The touch panel 350 includes an input device 375 and a display device370 that are provided to overlap with each other.

The input device 375 includes the substrate 162, an electrode 127, anelectrode 128, a plurality of wirings 137, and a plurality of wirings138. An FPC 72 b is electrically connected to each of the plurality ofwirings 137 and the plurality of wirings 138. An IC 73 b includes theFPC 72 b.

The display device 370 includes the substrate 51 and the substrate 61which are provided to face each other. The display device 370 includesthe display portion 62 and the driver circuit portion 64. The wiring 65and the like are provided over the substrate 51. An FPC 72 a iselectrically connected with the wiring 65. An IC 73 a is provided on theFPC 72 a.

The wiring 65 supplies signals and power to the display portion 62 andthe driver circuit portion 64. The signals and power are input to thewiring 65 from the outside or the IC 73 a, through the FPC 72 a.

FIG. 12 is a cross-sectional view of the pixel 900, the driver circuitportion 64, a region that includes the FPC 72 a, a region that includesthe FPC 72 b, and the like.

The substrates 51 and 61 are bonded to each other by the adhesive layer141. The substrates 61 and 162 are bonded to each other by an adhesivelayer 169. Here, the layers from the substrate 51 to the substrate 61correspond to the display device 370. The layers from the substrate 162to an electrode 124 correspond to the input device 375. That is, theadhesive layer 169 bonds the display device 370 and the input device 375together.

The structure of the display device 370 illustrated in FIG. 12 is astructure similar to the display device 100 illustrated in FIG. 6 ;detailed description is omitted here.

A polarizing plate 165 is bonded to the substrate 51 with an adhesivelayer 167. A backlight 161 is bonded to the polarizing plate 165 with anadhesive layer 163.

Examples of a type of backlight that can be used as the backlight 161include a direct-below backlight, an edge-light backlight and the like.The use of the direct-below backlight with light-emitting diodes (LEDs)is preferable as it enables complex local dimming and increase incontrast. The edge-light type backlight is preferably used because thethickness of a module including the backlight can be reduced. Moreover,quantum dots may be used for the backlight 161.

A quantum dot is a semiconductor nanocrystal with a size of severalnanometers and contains approximately 1×10³ to 1×10⁶ atoms. Since energyshift of quantum dots depends on their size, quantum dots made of thesame substance emit light with different wavelengths depending on theirsize; thus, emission wavelengths can be easily adjusted by changing thesize of quantum dots.

Since a quantum dot has an emission spectrum with a narrow peak,emission with high color purity can be obtained. In addition, a quantumdot is said to have a theoretical external quantum efficiency ofapproximately 100%, which far exceeds that of a fluorescent organiccompound, i.e., 25%, and is comparable to that of a phosphorescentorganic compound. Therefore, a quantum dot can be used as alight-emitting material to obtain a light-emitting element having highlight-emitting efficiency. Furthermore, since a quantum dot which is aninorganic compound has high inherent stability, a light-emitting elementwhich is favorable also in terms of lifetime can be obtained.

Examples of a material of a quantum dot include a Group 14 element inthe periodic table, a Group 15 element in the periodic table, a Group 16element in the periodic table, a compound of a plurality of Group 14elements in the periodic table, a compound of an element belonging toany of Groups 4 to 14 in the periodic table and a Group 16 element inthe periodic table, a compound of a Group 2 element in the periodictable and a Group 16 element in the periodic table, a compound of aGroup 13 element in the periodic table and a Group 15 element in theperiodic table, a compound of a Group 13 element in the periodic tableand a Group 17 element in the periodic table, a compound of a Group 14element in the periodic table and a Group 15 element in the periodictable, a compound of a Group 11 element in the periodic table and aGroup 17 element in the periodic table, iron oxides, titanium oxides,spinel chalcogenides, and semiconductor clusters.

Specific examples include, but are not limited to, cadmium selenide;cadmium sulfide; cadmium telluride; zinc selenide; zinc oxide; zincsulfide; zinc telluride; mercury sulfide; mercury selenide; mercurytelluride; indium arsenide; indium phosphide; gallium arsenide; galliumphosphide; indium nitride; gallium nitride; indium antimonide; galliumantimonide; aluminum phosphide; aluminum arsenide; aluminum antimonide;lead selenide; lead telluride; lead sulfide; indium selenide; indiumtelluride; indium sulfide; gallium selenide; arsenic sulfide; arsenicselenide; arsenic telluride; antimony sulfide; antimony selenide;antimony telluride; bismuth sulfide; bismuth selenide; bismuthtelluride; silicon; silicon carbide; germanium; tin; selenium;tellurium; boron; carbon; phosphorus; boron nitride; boron phosphide;boron arsenide; aluminum nitride; aluminum sulfide; barium sulfide;barium selenide; barium telluride; calcium sulfide; calcium selenide;calcium telluride; beryllium sulfide; beryllium selenide; berylliumtelluride; magnesium sulfide; magnesium selenide; germanium sulfide;germanium selenide; germanium telluride; tin sulfide; tin selenide; tintelluride; lead oxide; copper fluoride; copper chloride; copper bromide;copper iodide; copper oxide; copper selenide; nickel oxide; cobaltoxide; cobalt sulfide; triiron tetraoxide; iron sulfide; manganeseoxide; molybdenum sulfide; vanadium oxide; tungsten oxide; tantalumoxide; titanium oxide; zirconium oxide; silicon nitride; germaniumnitride; aluminum oxide; barium titanate; a compound of selenium, zinc,and cadmium; a compound of indium, arsenic, and phosphorus; a compoundof cadmium, selenium, and sulfur; a compound of cadmium, selenium, andtellurium; a compound of indium, gallium, and arsenic; a compound ofindium, gallium, and selenium; a compound of indium, selenium, andsulfur; a compound of copper, indium, and sulfur; and combinationsthereof. What is called an alloyed quantum dot, whose composition isrepresented by a given ratio, may be used. For example, an alloyedquantum dot of cadmium, selenium, and sulfur is a means effective inobtaining blue light because the emission wavelength can be changed bychanging the content ratio of elements.

As the quantum dot, any of a core-type quantum dot, a core-shell quantumdot, a core-multishell quantum dot, and the like can be used. Note thatwhen a core is covered with a shell formed of another inorganic materialhaving a wider band gap, the influence of defects and dangling bondsexisting at the surface of a nanocrystal can be reduced. Since such astructure can significantly improve the quantum efficiency of lightemission, it is preferable to use a core-shell or core-multishellquantum dot. Examples of the material of a shell include zinc sulfideand zinc oxide.

Quantum dots have a high proportion of surface atoms and thus have highreactivity and easily cohere together. For this reason, it is preferablethat a protective agent be attached to, or a protective group beprovided at the surfaces of quantum dots. The attachment of theprotective agent or the provision of the protective group can preventcohesion and increase solubility in a solvent. It can also reducereactivity and improve electrical stability. Examples of the protectiveagent (or the protective group) include polyoxyethylene alkyl etherssuch as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, andpolyoxyethylene oleyl ether; trialkylphosphines such astripropylphosphine, tributylphosphine, trihexylphosphine, andtrioctylphoshine; polyoxyethylene alkylphenyl ethers such aspolyoxyethylene n-octylphenyl ether and polyoxyethylene n-nonylphenylether; tertiary amines such as tri(n-hexyl)amine, tri(n-octyl)amine, andtri(n-decyl)amine; organophosphorus compounds such as tripropylphosphineoxide, tributylphosphine oxide, trihexylphosphine oxide,trioctylphosphine oxide, and tridecylphosphine oxide; polyethyleneglycol diesters such as polyethylene glycol dilaurate and polyethyleneglycol distearate; organic nitrogen compounds such asnitrogen-containing aromatic compounds, e.g., pyridines, lutidines,collidines, and quinolines; aminoalkanes such as hexylamine, octylamine,decylamine, dodecylamine, tetradecylamine, hexadecylamine, andoctadecylamine; dialkylsulfides such as dibutylsulfide;dialkylsulfoxides such as dimethylsulfoxide and dibutylsulfoxide;organic sulfur compounds such as sulfur-containing aromatic compounds,e.g., thiophenes; higher fatty acids such as a palmitin acid, a stearicacid, and an oleic acid; alcohols; sorbitan fatty acid esters; fattyacid modified polyesters; tertiary amine modified polyurethanes; andpolyethyleneimines.

Since band gaps of quantum dots are increased as their size isdecreased, the size is adjusted as appropriate so that light with adesired wavelength can be obtained. Light emission from the quantum dotsis shifted to a blue color side, i.e., a high energy side, as thecrystal size is decreased; thus, emission wavelengths of the quantumdots can be adjusted over wavelength regions of spectra of anultraviolet region, a visible light region, and an infrared region bychanging the size of quantum dots. The range of size (diameter) ofquantum dots which is usually used is 0.5 nm to 20 nm, preferably 1 nmto 10 nm. The emission spectra are narrowed as the size distribution ofthe quantum dots gets smaller, and thus light can be obtained with highcolor purity. The shape of the quantum dots is not particularly limitedand may be a spherical shape, a rod shape, a circular shape, or thelike. Quantum rods which are rod-like shape quantum dots emitdirectional light polarized in the c-axis direction; thus, quantum rodscan be used as a light-emitting material to obtain a light-emittingelement with higher external quantum efficiency.

A polarizing plate 166 is bonded to the substrate 162 by an adhesivelayer 168. A protection substrate 160 is bonded to the polarizing plate166 by an adhesive layer 164. The protection substrate 160 may be usedas the substrate that objects such as a finger or a stylus directlycontact, when the touch panel 350 is incorporated into an electronicdevice. A substrate that can be used as the substrates 51 and 61 or thelike can be used as the protection substrate 160. A structure where aprotective layer is formed on the surface of the substrate that can beused as the substrates 51 and 61 or the like is preferably used for theprotection substrate 160. Alternatively, a reinforced glass or the likeis preferably used as the protection substrate 160. The protective layercan be formed with a ceramic coating. The protective layer can be formedusing an inorganic insulating material such as silicon oxide, aluminumoxide, yttrium oxide, or yttria-stabilized zirconia (YSZ).

The polarizing plate 166 may be provided between the input device 375and the display device 370. In that case, the protection substrate 160,the adhesive layer 164, and the adhesive layer 168 that are illustratedin FIG. 12 are not necessarily provided. In other words, the substrate162 can be positioned on the outermost surface of the touch panel 350.The above-described material that can be used for the protectionsubstrate 160 is preferably used for the substrate 162.

The electrodes 127 and 128 are provided over the substrate 162, on thesubstrate 61 side. The electrodes 127 and 128 are formed on the sameplane. An insulating layer 125 is provided to cover the electrodes 127and 128. The electrode 124 is electrically connected to two of theelectrodes 128 that are provided on both sides of the electrode 127,through an opening provided in the insulating layer 125.

In the conductive layers included in the input device 375, theconductive layers (e.g., the electrodes 127 and 128) that overlap withthe display region 918 are formed using a visible-light-transmittingmaterial.

The wiring 137 that is obtained by processing the same conductive layeras the electrodes 127 and 128 is connected to a conductive layer 126that is obtained by processing the same conductive layer as theelectrode 124. The conductive layer 126 is electrically connected to theFPC 72 b through a connector 242 b.

This embodiment can be combined with any of other embodiments asappropriate. In the case where a plurality of structure examples aredescribed in one embodiment in this specification, some of the structureexamples can be combined as appropriate.

Embodiment 2

In this embodiment, operation modes which can be executed in the displaydevice of one embodiment of the present invention are described withreference to FIGS. 13A to 13C.

A normal driving mode (Normal mode) with a normal frame frequency(typically, higher than or equal to 60 Hz and lower than or equal to 240Hz) and an idling stop (IDS) driving mode with a low frame frequencywill be described below.

Note that the IDS driving mode refers to a driving method in which afterimage data is written, rewriting of image data is stopped. Thisincreases the interval between writing of image data and subsequentwriting of image data, thereby reducing the power that would be consumedby writing of image data in that interval. The IDS driving mode can beperformed at a frame frequency which is 1/100 to 1/10 of the normaldriving mode, for example. A still image is displayed by the same videosignals in consecutive frames. Thus, the IDS driving mode isparticularly effective when displaying a still image. When an image isdisplayed using IDS driving, power consumption is reduced, imageflickering is suppressed, and eyestrain can be reduced.

FIG. 13A shows a pixel circuit and FIGS. 13B and 13C are timing chartsshowing the normal driving method and the IDS driving mode. Note that inFIG. 13A, a first display element 501 (here, a reflective liquid crystalelement) and a pixel circuit 506 electrically connected to the firstdisplay element 501 are illustrated. In the pixel circuit 506illustrated in FIG. 13A, a signal line SL, a gate line GL, a transistorM1 connected to the signal line SL and the gate line GL, and a capacitorC_(SLC) connected to the transistor M1 are illustrated. Note that a nodeto which one of electrodes of the first display element 501, one of asource and a drain of the transistor M1, and the capacitor C_(SLC) areconnected is referred to as a node ND1.

The transistor M1 can serve as a leakage path of data Di retained in thecapacitor C_(SLC). The off-state current of the transistor M1 ispreferably as low as possible. A transistor including a metal oxide in asemiconductor layer in which a channel is formed is preferably used asthe transistor M1. A metal oxide having at least one of an amplificationfunction, a rectification function, and a switching function can bereferred to as a metal oxide semiconductor or an oxide semiconductor(abbreviated to an OS). As a typical example of a transistor, atransistor including an oxide semiconductor in a semiconductor layer inwhich a channel is formed (OS transistor) is described. The OStransistor has a feature of extremely low leakage current in anon-conduction state (extremely low off-state current) as compared to atransistor containing silicon in its semiconductor layer (Sitransistor). The use of an OS transistor as the transistor M1 makes itpossible to retain charge supplied to the node ND1 for a long time.

Note that in the circuit diagram illustrated in FIG. 13A, the liquidcrystal element LC becomes a leakage path of the data Di. Therefore, toperform IDS driving appropriately, the resistivity of the liquid crystalelement LC is preferably higher than or equal to 1.0×10¹⁴ Ω·cm.

Note that for example, an In-Ga—Zn oxide or an In—Zn oxide is preferablyused for a channel region of the above OS transistor. The In-Ga—Zn oxidecan typically have an atomic ratio of In:Ga:Zn=4:2:4.1 or a neighborhoodthereof.

FIG. 13B is a timing chart showing waveforms of signals supplied to thesignal line SL and the gate line GL in the normal driving mode. In thenormal driving mode, a normal frame frequency (e.g., 60 Hz) is used foroperation. Here, periods T1 to T3 each denote one frame period; in eachframe period, a scanning signal is supplied to the gate line GL, anddata Di is written from the signal line SL to the node ND1. Thisoperation is also performed to write the same data Di in the periods T1to T3 and to write different data in the periods T1 to T3.

FIG. 13C is a timing chart showing waveforms of signals supplied to thesignal line SL and the gate line GL in the IDS driving mode. In the IDSdriving, a low frame frequency (e.g., 1 Hz) is used for operation. Oneframe period is denoted by a period T1 and includes a data writingperiod T_(W) and a data retention period T_(RET). In the IDS drivingmode, a scanning signal is supplied to the gate line GL and the data Diof the signal line SL is written to the capacitor C_(SLC) in the periodT_(W), the gate line GL is fixed to a low-level voltage in the periodT_(RET), and the transistor M1 is turned off so that the written data Diis retained in the capacitor C_(SLC). Note that the low frame frequencymay be higher than or equal to 0.1 Hz and lower than 60 Hz, for example.For example, the low frame frequency may be higher than or equal to 0.1Hz and lower than 20 Hz.

This embodiment can be combined with any of other embodiments asappropriate.

Embodiment 3

In this embodiment, an example of a method for driving a touch sensor isdescribed with reference to drawings.

<Example of Sensing Method of Sensor>

FIG. 14A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 14A illustrates a pulse voltage outputcircuit 551 and a current sensing circuit 552. In FIG. 14A, as anexample, six wirings X1 to X6 represent electrodes 521 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent electrodes 522that sense changes in current. FIG. 14A also illustrates a capacitor 553that is formed where the electrodes 521 and 522 overlap with each other.Note that functional replacement between the electrodes 521 and 522 ispossible.

The pulse voltage output circuit 551 is a circuit for sequentiallyapplying pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 521 and 522 of the capacitor 553. When theelectric field between the wirings is shielded, for example, a changeoccurs in the capacitor 553 (mutual capacitance). Proximity or a touchof an object can be sensed by utilizing this change.

The current sensing circuit 552 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by changesin mutual capacitance of the capacitors 553. No change in current valueis detected in the wirings Y1 to Y6 when there is no approach or contactof an object, whereas a decrease in current value is detected whenmutual capacitance is decreased owing to the approach or contact of anobject. Note that an integrator circuit or the like is used for sensingof current.

Note that one or both of the pulse voltage output circuit 551 and thecurrent sensing circuit 552 may be provided over the substrate 51 or thesubstrate 61 illustrated in FIG. 5 or the like. For example, it ispreferable that one or both of the pulse voltage output circuit 551 andthe current sensing circuit 552 be formed in the same process as that ofthe display portion 62, the driver circuit portion 64, or the likebecause the process can be simplified and the number of components usedfor driving the touch sensor can be reduced. Alternatively, one or bothof the pulse voltage output circuit 551 and the current sensing circuit552 may be mounted on the IC 73.

In particular, when the semiconductor layers where channels of thetransistors over the substrate 51 are formed using crystalline siliconsuch as polycrystalline silicon or single crystal silicon, drivingcharacteristics of the pulse voltage output circuit 551, the currentsensing circuit 552, and the like are increased and sensitivity of thetouch sensor can be thus increased.

FIG. 14B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 14A. In FIG. 14B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 14B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change uniformly in accordance withchanges in the voltages of the wirings X1 to X6. The current value isdecreased at the point of approach or contact of a sensing target andaccordingly the waveform of the voltage value changes.

By sensing a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<Example of Driving Method of Display Device>

FIG. 15A is a block diagram illustrating a configuration example of adisplay device. FIG. 15A illustrates a gate driver circuit GD (a scanline driver circuit), a source driver circuit SD (a signal line drivercircuit), and a display portion including a plurality of pixels pix. InFIG. 15A, gate lines x_1 to x_m (m is a natural number) electricallyconnected to the gate driver circuit GD and source lines y_1 to y_n (nis a natural number) electrically connected to the source driver circuitSD are shown. Corresponding to these lines, the pixels pix are denotedby (1, 1) to (n, m).

FIG. 15B is a timing chart of signals supplied to the gate lines and thesource lines in the display device shown in FIG. 15A. The periods inFIG. 15B show the case where data signals are rewritten every frameperiod and the case where data signals are not rewritten. Note thatperiods such as a retrace period are not taken into consideration inFIG. 15B.

In the case where data signals are rewritten every frame period, scansignals are sequentially supplied to the gate lines x_1 to x_m. In ahorizontal scanning period 1H, during which the scan signal is at an Hlevel, data signals D are supplied to the source lines y_l to y_n in thecolumns.

In the case where data signals are not rewritten every frame period,supply of scan signals to the gate lines x_l to x_m is stopped. In thehorizontal scanning period 1H, supply of data signals to the sourcelines y_l to y_n in the columns is stopped.

A driving method in which data signals are not rewritten every frameperiod is effective particularly when an oxide semiconductor is used forthe semiconductor layer where a channel is formed in the transistorincluded in the pixel pix. A transistor including an oxide semiconductorcan have much lower off-state current than a transistor including asemiconductor such as silicon. Thus, a data signal written in theprevious period can be held without rewriting data signals every frameperiod, and grayscale of pixels can be held for 1 second or longer,preferably 5 seconds or longer, for example.

In the case where polycrystalline silicon or the like is used for asemiconductor layer where a channel of a transistor included in thepixel pix is formed, the storage capacitance of the pixel is preferablyincreased in advance. The larger the storage capacitance is, the longerthe grayscale of the pixel can be held. The storage capacitance may bedetermined depending on leakage current of a transistor or a displayelement which is electrically connected to the storage capacitor. Forexample, the storage capacitance per pixel is set to 5 fF to 5 pFinclusive, preferably 10 fF to 5 pF inclusive, further preferably 20 fFto 1 pF inclusive, so that a data signal written in the previous periodcan be held without rewriting data signals every frame period. Forexample, grayscale of a pixel can be held for several frame periods orseveral tens of frame periods.

<Example of Driving Method of Display Portion and Touch Sensor>

FIGS. 16A to 16D show examples of the operations in successive frameperiods of the touch sensor described with reference to FIGS. 14A and14B and the display portion described with reference to FIGS. 15A and15B that are driven for 1 sec (one second). In FIG. 16A, one frameperiod for the display portion is 16.7 ms (frame frequency: 60 Hz), andone frame period for the touch sensor is 16.7 ms (frame frequency: 60Hz).

In the display device of one embodiment of the present invention, thedisplay portion and the touch sensor operate independently of eachother, and the display device can have a touch sensing period concurrentwith a display period. That is why one frame period for the displayportion and one frame period for the touch sensor can both be 16.7 ms(frame frequency: 60 Hz) as shown in FIG. 16A. The frame frequency forthe touch sensor may differ from that of the display portion. Forexample, as shown in FIG. 16B, one frame period for the display portionmay be 8.3 ms (frame frequency: 120 Hz) and one frame period for thetouch sensor may be 16.7 ms (frame frequency: 60 Hz). The framefrequency for the display portion may be 33.3 ms (frame frequency: 30Hz) (not shown).

The frame frequency for the display portion may be changeable, i.e., theframe frequency in displaying moving images may be increased (e.g., 60Hz or more, or 120 Hz or more), whereas the frame frequency indisplaying still images may be decreased (e.g., 60 Hz or less, 30 Hz orless, or 1 Hz or less). With this structure, power consumption of thedisplay device can be reduced. The frame frequency for the touch sensormay be changeable, and the frame frequency in waiting may differ fromthe frame frequency in sensing a touch.

Moreover, in the display device of one embodiment of the presentinvention, the following operation is possible: data signals are notrewritten in the display portion and a data signal written in theprevious period is held. In that case, one frame period of the displayportion can be longer than 16.7 ms. Thus, as shown in FIG. 16C, theoperation can be switched so that one frame period for the displayportion is 1 sec (frame frequency: 1 Hz) and one frame period for thetouch sensor is 16.7 ms (frame frequency: 60 Hz).

Note that for the operation in which data signals are not rewritten inthe display portion and a data signal written in the previous period isheld, the above-described IDS driving mode can be referred to. As theIDS driving mode, a partial IDS driving mode may be employed in whichdata signals are rewritten only in a specific region of the displayportion. The partial IDS driving mode is a mode in which data signalsare rewritten only in a specific region of the display portion and adata signal written in the previous period is held in the other region.

Furthermore, by the driving method of a touch sensor that is disclosedin this embodiment, the touch sensor can be continuously driven in thecase of FIG. 16C. Thus, data signals in the display portion can also berewritten when the approach or contact of a sensing target is sensed bythe touch sensor, as shown in FIG. 16D.

If rewriting of data signals in a display portion is performed during asensing period of a touch sensor, noise caused by rewriting of the datasignals travels through the touch sensor and the sensitivity of thetouch sensor might decrease. For this reason, rewriting of data signalsin a display portion and sensing by a touch sensor are preferablyperformed in different periods.

FIG. 17A shows an example in which rewriting of data signals in adisplay portion and sensing by a touch sensor are performed alternately.FIG. 17B shows an example in which sensing by a touch sensor isperformed one time every two rewritings of data signals in a displayportion. Note that sensing by a touch sensor may be performed once everythree or more rewritings.

In the case where an oxide semiconductor is used in a semiconductorlayer (where a channel is formed) of a transistor used in the pixel pix,the off-state current can be significantly reduced and the frequency ofrewriting data signal can be sufficiently reduced. Specifically, asufficiently long break period can be set between rewritings of datasignals. The break period can be 0.5 seconds or longer, 1 second orlonger, or 5 seconds or longer, for example. The upper limit of thebreak period depends on leakage current of a capacitor or a displayelement connected to a transistor; for example, 1 minute or shorter, 10minutes or shorter, 1 hour or shorter, or 1 day or shorter.

FIG. 17C shows an example in which rewriting of data signals in adisplay portion is performed once every 5 seconds. A break period forstopping the rewriting operation of a display portion is set in FIG. 17Cbetween rewriting of data signals and next rewriting. In the breakperiod, a touch sensor can be operated at a frame frequency of i Hz (iis more than or equal to the frame frequency of a display device; here,0.2 Hz or more). Sensing by a touch sensor is performed in a breakperiod and is not performed in a rewriting period of data signals in adisplay portion as shown in FIG. 17C, so that sensitivity of a touchsensor can be increased. When rewriting of data signals in a displayportion and sensing by a touch sensor are performed at the same time asshown in FIG. 17D, operation signals can be simplified.

In a break period during which rewriting of data signals in a displayportion is not performed, not only the supply of data signals to thedisplay portion, but also the operation of one or both of the gatedriver circuit GD and the source driver circuit SD may be stopped. Thesupply of power to one or both of the gate driver circuit GD and thesource driver circuit SD may also be stopped. Thus, noise is furtherreduced, and the sensitivity of the touch sensor can be furtherincreased. Moreover, the power consumption of the display device can befurther reduced.

The display device of one embodiment of the present invention includes adisplay portion and a touch sensor between two substrates. With thisstructure, the distance between the display portion and the touch sensorcan be reduced. At this time, noise is easily transmitted to the touchsensor in driving the display portion, which might reduce thesensitivity of the touch sensor. When the driving method in thisembodiment is employed, a display device including a touch sensor, whichhas both reduced thickness and high sensitivity, can be obtained.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

Described in this embodiment is a metal oxide that can be used in asemiconductor layer of a transistor disclosed in one embodiment of thepresent invention. Note that in the case where a metal oxide is used ina semiconductor layer of a transistor, the metal oxide can be rephrasedas an oxide semiconductor.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofthe non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

For the semiconductor layer of the transistor disclosed in oneembodiment of the present invention, a cloud-aligned composite oxidesemiconductor (CAC-OS) may be used.

The aforementioned non-single-crystal oxide semiconductor or CAC-OS canbe suitably used in a semiconductor layer of a transistor disclosed inone embodiment of the present invention. As the non-single-crystal oxidesemiconductor, an nc-OS or a CAAC-OS can be suitably used.

In one embodiment of the present invention, a CAC-OS is preferably usedin a semiconductor layer of a transistor. The use of the CAC-OS allowsthe transistor to have high electrical characteristics or highreliability.

The CAC-OS will be described in detail below.

A CAC-OS or a CAC metal oxide has a conducting function in a part of thematerial and has an insulating function in another part of the material;as a whole, the CAC-OS or the CAC metal oxide has a function of asemiconductor. In the case where the CAC-OS or the CAC metal oxide isused in a channel region of a transistor, the conducting function is toallow electrons (or holes) serving as carriers to flow, and theinsulating function is to not allow electrons serving as carriers toflow. By the complementary action of the conducting function and theinsulating function, the CAC-OS or the CAC metal oxide can have aswitching function (on/off function). In the CAC-OS or the CAC metaloxide, separation of the functions can maximize each function.

The CAC-OS or the CAC metal oxide includes conductive regions andinsulating regions. The conductive regions have the aforementionedconducting function and the insulating regions have the aforementionedinsulating function. In some cases, the conductive regions and theinsulating regions in the material are separated at the nanoparticlelevel. In some cases, the conductive regions and the insulating regionsare unevenly distributed in the material. The conductive regions aresometimes observed to be coupled in a cloud-like manner with theirboundaries blurred.

In the CAC-OS or the CAC metal oxide, the conductive regions and theinsulating regions each have a size greater than or equal to 0.5 nm andless than or equal to 10 nm, preferably greater than or equal to 0.5 nmand less than or equal to 3 nm and are dispersed in the material, insome cases.

The CAC-OS or the CAC metal oxide includes components having differentbandgaps. For example, the CAC-OS or the CAC metal oxide includes acomponent having a wide gap due to the insulating region and a componenthaving a narrow gap due to the conductive region. In the case of such acomposition, carriers mainly flow in the component having a narrow gap.The component having a narrow gap complements the component having awide gap, and carriers also flow in the component having a wide gap inconjunction with the component having a narrow gap. Therefore, in thecase where the above-described CAC-OS or CAC metal oxide is used in achannel region of a transistor, high current drive capability in the onstate of the transistor, that is, high on-state current and highfield-effect mobility, can be obtained.

In other words, the CAC-OS or the CAC-metal oxide can be referred to asa matrix composite or a metal matrix composite.

The CAC-OS has, for example, a composition in which elements included ina metal oxide are unevenly distributed. The unevenly distributedelements each have a size greater than or equal to 0.5 nm and less thanor equal to 10 nm, preferably greater than or equal to 1 nm and lessthan or equal to 2 nm, or a similar size. Note that in the followingdescription of a metal oxide, a state in which one or more metalelements are unevenly distributed and regions including the metalelement(s) are mixed is referred to as a mosaic pattern or a patch-likepattern. The regions each have a size greater than or equal to 0.5 nmand less than or equal to 10 nm, preferably greater than or equal to 1nm and less than or equal to 2 nm, or a similar size.

Note that a metal oxide preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition, oneor more elements selected from aluminum, gallium, yttrium, copper,vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, magnesium, and the like may be contained.

For example, of the CAC-OS, an In-Ga—Zn oxide with the CAC composition(such an In-Ga—Zn oxide may be particularly referred to as CAC-IGZO) hasa composition in which materials are separated into indium oxide(InO_(X1), where X1 is a real number greater than 0) or indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0) or gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4,Y4, and Z4 are real numbers greater than 0), and a mosaic pattern isformed. Then, InO_(x1) or In_(X2)Zn_(Y2)O_(Z2) forming the mosaicpattern is evenly distributed in the film. This composition is alsoreferred to as a cloud-like composition.

That is, the CAC-OS is a composite metal oxide with a composition inwhich a region including GaO_(X3) as a main component and a regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component aremixed. Note that in this specification, for example, when the atomicratio of In to an element M in a first region is greater than the atomicratio of In to an element M in a second region, the first region hashigher In concentration than the second region.

Note that a compound including In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a c-axis-aligned crystalline (CAAC)structure. Note that the CAAC structure is a crystal structure in whicha plurality of IGZO nanocrystals have c-axis alignment and are connectedin the a-b plane direction without alignment.

On the other hand, the CAC-OS relates to the material composition of ametal oxide. In a material composition of a CAC-OS including In, Ga, Zn,and O, nanoparticle regions including Ga as a main component areobserved in part of the CAC-OS and nanoparticle regions including In asa main component are observed in part thereof. These nanoparticleregions are randomly dispersed to form a mosaic pattern. Therefore, thecrystal structure is a secondary element for the CAC-OS.

Note that in the CAC-OS, a stacked-layer structure including two or morefilms with different atomic ratios is not included. For example, atwo-layer structure of a film including In as a main component and afilm including Ga as a main component is not included.

A boundary between the region including GaO_(X3) as a main component andthe region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases.

In the case where one or more of aluminum, yttrium, copper, vanadium,beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like are contained instead of gallium in a CAC-OS,nanoparticle regions including the selected metal element(s) as a maincomponent(s) are observed in part of the CAC-OS and nanoparticle regionsincluding In as a main component are observed in part thereof, and thesenanoparticle regions are randomly dispersed to form a mosaic pattern inthe CAC-OS.

The CAC-OS can be formed by a sputtering method under conditions where asubstrate is not heated intentionally, for example. In the case offorming the CAC-OS by a sputtering method, one or more selected from aninert gas (typically, argon), an oxygen gas, and a nitrogen gas may beused as a deposition gas. The ratio of the flow rate of an oxygen gas tothe total flow rate of the deposition gas at the time of deposition ispreferably as low as possible, and for example, the flow ratio of anoxygen gas is preferably higher than or equal to 0% and less than 30%,further preferably higher than or equal to 0% and less than or equal to10%.

The CAC-OS is characterized in that no clear peak is observed inmeasurement using θ/2θ scan by an out-of-plane method, which is an X-raydiffraction (XRD) measurement method. That is, X-ray diffraction showsno alignment in the a-b plane direction and the c-axis direction in ameasured region.

In an electron diffraction pattern of the CAC-OS which is obtained byirradiation with an electron beam with a probe diameter of 1 nm (alsoreferred to as a nanometer-sized electron beam), a ring-like region withhigh luminance and a plurality of bright spots in the ring-like regionare observed. Therefore, the electron diffraction pattern indicates thatthe crystal structure of the CAC-OS includes a nanocrystal (nc)structure with no alignment in plan-view and cross-sectional directions.

For example, an energy dispersive X-ray spectroscopy (EDX) mapping imageconfirms that an In-Ga—Zn oxide with the CAC composition has a structurein which a region including GaO_(X3) as a main component and a regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component areunevenly distributed and mixed.

The CAC-OS has a structure different from that of an IGZO compound inwhich metal elements are evenly distributed, and has characteristicsdifferent from those of the IGZO compound. That is, in the CAC-OS,regions including GaO_(X3) or the like as a main component and regionsincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component areseparated to form a mosaic pattern.

The conductivity of a region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component is higher than that of a region including GaO_(X3)or the like as a main component. In other words, when carriers flowthrough regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent, the conductivity of an oxide semiconductor is exhibited.Accordingly, when regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) asa main component are distributed in an oxide semiconductor like a cloud,high field-effect mobility (μ) can be achieved.

In contrast, the insulating property of a region including GaO_(X3) orthe like as a main component is higher than that of a region includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. In other words,when regions including GaO_(X3) or the like as a main component aredistributed in an oxide semiconductor, leakage current can be suppressedand favorable switching operation can be achieved.

Accordingly, when a CAC-OS is used for a semiconductor element, theinsulating property derived from GaO_(X3) or the like and theconductivity derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complementeach other, whereby high on-state current (Ion) and high field-effectmobility (μ) can be achieved.

A semiconductor element including a CAC-OS has high reliability. Thus,the CAC-OS is suitably used in a variety of semiconductor devicestypified by a display.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In this embodiment, electronic devices of one embodiment of the presentinvention are described.

Examples of electronic devices include a television set, a desktop ornotebook personal computer, a monitor of a computer or the like, adigital camera, a digital video camera, a digital photo frame, a mobilephone, a portable game machine, a portable information terminal, anaudio reproducing device, and a large game machine such as a pachinkomachine.

FIG. 18A illustrates a notebook personal computer which includes ahousing 8111, a display portion 8112, a keyboard 8113, a pointing device8114, and the like.

The display device of one embodiment of the present invention can beused in the display portion 8112. Accordingly, the notebook personalcomputer having the display portion with a high aperture ratio can beprovided.

FIGS. 18B and 18C illustrate examples of digital signages. The digitalsignages each include a housing 8000, a display portion 8001, a speaker8003, and the like. Also, the digital signages can each include an LEDlamp, operation keys (including a power switch or an operation switch),a connection terminal, a variety of sensors, a microphone, and the like.

FIG. 18C illustrates a digital signage mounted on a cylindrical pillar.

The display device of one embodiment of the present invention can beused in the display portion 8001 illustrated in FIGS. 18B and 18C.Accordingly, the digital signage having the display portion with a highaperture ratio can be provided.

A larger display portion 8001 can provide more information at a time. Inaddition, a larger display portion 8001 attracts more attention, so thatthe effectiveness of the advertisement is expected to be increased, forexample.

It is preferable to use a touch panel in the display portion 8001because a device with such a structure does not just display a still ormoving image, but can be operated by users intuitively. Alternatively,in the case where the display device of one embodiment of the presentinvention is used for providing information such as route information ortraffic information, usability can be enhanced by intuitive operation.

FIG. 18D is an external view of an automobile 7900. FIG. 18E illustratesa driver's seat of the automobile 7900. The automobile 7900 includes acar body 7901, wheels 7902, a windshield 7903, lights 7904, fog lamps7905, and the like.

The display device of one embodiment of the present invention can beused in a display portion and the like of the automobile 7900. Forexample, the display device of one embodiment of the present inventioncan be used in display portions 7910 to 7917 illustrated in FIG. 18E.Accordingly, the automobile having the display portions with a highaperture ratio can be provided.

The display portion 7910 and the display portion 7911 are provided inthe windshield 7903 of the automobile 7900. The display device of oneembodiment of the present invention can be a see-through device, throughwhich the opposite side can be seen, by using a light-transmittingconductive material for its electrodes. Such a see-through displaydevice does not hinder driver's vision during the driving of theautomobile 7900. Thus, the display device 10 of one embodiment of thepresent invention can be provided in the windshield 7903 of theautomobile 7900. Note that in the case where a transistor or the like isprovided in the display device, a transistor having light-transmittingproperties, such as an organic transistor using an organic semiconductormaterial or a transistor using an oxide semiconductor, is preferablyused.

A display portion 7912 is provided on a pillar portion. A displayportion 7913 is provided on a dashboard. A display portion 7914 isprovided in a door portion. For example, the display portion 7912 cancompensate for the view hindered by the pillar portion by showing animage taken by an imaging unit provided on the car body. Similarly, thedisplay portion 7913 can compensate for the view hindered by thedashboard and the display portion 7914 can compensate for the viewhindered by the door. That is, showing an image taken by an imaging unitprovided on the outside of the car body leads to elimination of blindareas and enhancement of safety. In addition, showing an image so as tocompensate for the area which a driver cannot see makes it possible forthe driver to confirm safety easily and comfortably.

The display portion 7917 is provided in a steering wheel. The displayportion 7915, the display portion 7916, or the display portion 7917 candisplay a variety of information such as navigation information, aspeedometer, a tachometer, a mileage, a fuel meter, a gearshiftindicator, and air-condition setting. The content, layout, or the likeof the display on the display portions can be changed freely by a useras appropriate. The information listed above can also be displayed onthe display portions 7910 to 7914.

The display portions 7910 to 7917 can also be used as lighting devices.

This embodiment can be combined with any of other embodiments asappropriate.

This application is based on Japanese Patent Application Serial No.2016-226897 filed with Japan Patent Office on Nov. 22, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a transistorcomprising: a first insulating layer; a first semiconductor layercomprising a channel region over and in contact with the firstinsulating layer; a second insulating layer over the first semiconductorlayer; and a first conductive layer directly connected to the firstsemiconductor layer via an opening portion in the second insulatinglayer; and a capacitor comprising: a second conductive layer over and incontact with the first insulating layer; the second insulating layerover the second conductive layer; and the first conductive layer overthe second insulating layer, wherein the first conductive layer servesas one electrode of the capacitor and one of a source electrode and adrain electrode of the transistor, wherein the second insulating layerserves as a dielectric layer of the capacitor and a channel protectivelayer of the transistor, and wherein the first conductive layer and thesecond conductive layer comprise a metal oxide and have alight-transmitting property.
 2. The display device according to claim 1,wherein the metal oxide comprises indium and zinc.
 3. The display deviceaccording to claim 1, wherein the second insulating layer comprisessilicon and nitrogen.
 4. The display device according to claim 1,further comprising a third insulating layer over and in contact with thesecond insulating layer and the first conductive layer, wherein thethird insulating layer comprises oxygen at a higher proportion than astoichiometric composition.
 5. The display device according to claim 1,further comprising a gate electrode of the transistor having alight-blocking property.
 6. The display device according to claim 1,wherein the other of the source electrode and the drain electrode has alight-blocking property.
 7. A display device comprising: a transistorcomprising: a first insulating layer; a first semiconductor layercomprising a channel region over and in contact with the firstinsulating layer; a second insulating layer over the first semiconductorlayer; and a first conductive layer directly connected to the firstsemiconductor layer via an opening portion in the second insulatinglayer; and a capacitor comprising: a second conductive layer over and incontact with the first insulating layer; the second insulating layerover the second conductive layer; and the first conductive layer overthe second insulating layer, wherein the first conductive layer and thesecond conductive layer comprise a metal oxide and have alight-transmitting property.
 8. The display device according to claim 7,wherein the metal oxide comprises indium and zinc.
 9. The display deviceaccording to claim 7, wherein the second insulating layer comprisessilicon and nitrogen.
 10. The display device according to claim 7,further comprising a third insulating layer over and in contact with thesecond insulating layer and the first conductive layer, wherein thethird insulating layer comprises oxygen at a higher proportion than astoichiometric composition.
 11. The display device according to claim 7,further comprising a gate electrode of the transistor having alight-blocking property.
 12. The display device according to claim 7,wherein the first conductive layer serves as one of a source electrodeand a drain electrode of the transistor, and wherein the other of thesource electrode and the drain electrode has a light-blocking property.13. A display device comprising: a transistor comprising: a firstinsulating layer; a first semiconductor layer comprising a channelregion over and in contact with the first insulating layer; a secondinsulating layer over the first semiconductor layer; and a firstconductive layer electrically connected to the first semiconductor layervia an opening portion in the second insulating layer; and a capacitorcomprising: a second conductive layer over and in contact with the firstinsulating layer; the second insulating layer over the second conductivelayer; and the first conductive layer over the second insulating layer,wherein the first conductive layer and the second conductive layercomprise a metal oxide and have a light-transmitting property, whereinthe first conductive layer serves as one of a source electrode and adrain electrode of the transistor, and wherein the other of the sourceelectrode and the drain electrode has a light-blocking property.
 14. Thedisplay device according to claim 13, wherein the metal oxide comprisesindium and zinc.
 15. The display device according to claim 13, whereinthe second insulating layer comprises silicon and nitrogen.
 16. Thedisplay device according to claim 13, further comprising a thirdinsulating layer over and in contact with the second insulating layerand the first conductive layer, wherein the third insulating layercomprises oxygen at a higher proportion than a stoichiometriccomposition.
 17. The display device according to claim 13, furthercomprising a gate electrode of the transistor having a light-blockingproperty.